So, this article is going to talk about the usefulness of using your surroundings to get a good recording environment, and will also explain microphones, what they are, use what for which situation, etc.
You'll want to start off with your:
You got to settle on where you want to do most of your recordings (in essence, you can record anywhere, as any place on earth gives a different sound) which will most likely be your bedroom or spare room or something along those lines. Most of these rooms are squarish or a cuboid in shape, which isn't the best place to have reflective surfaces.
And this is basically the bit you need to concern yourself about when it comes to your actual room. The reflective surfaces essentially make every bit of the sound of a sound source change in smallish ways but the overall effect is noticeable. Having an irregularly shaped room (say, with 5 walls instead of 4) is actually more helpful than hindering, as there is more surface for the sound to reflect off of, and having different leveled ceilings also helps. A simple solution (but very expensive one) is to have your room of choice renovated/converted into a designed sound room (an interesting room shape is to have a sort of trumpet funnel shape where your main work area is). Then again, very expensive, always alternatives.
I don't really want to explain the principles of sound, that's more of an in depth science explanation I don't have the answer for at the minute, but a basic explanation would be that it works in a similar manner to light I.e. It likes to bounce off of things with a certain reflective property. So for light = white, shiny surface and sound = hard, dense surface. Rooms with carpets can be difficult to work with at times.
Another concern is having too many surfaces. This problem can occur from just having too much stuff in a room. Making sure the main pathway for the sound of your sound source is clear of objects. Another problem is windows and doors. Although there's not much you can really do about them being there, try to place your sound source in such a way so that it doesn't directly or almost instantly hit a window or door. The glass in windows is a lot thinner than walls, so most sound waves pass through them very easily, and doors are a source of extra corners, which trap bass frequencies and may reduce the strength of your sound.
So there's two types of room you can do for recording: A reflective room or a dead room. A reflective room is essentially just your room as a natural reverb unit, using it to get a full, atmospheric and interesting sound depending on what you're using it for (great for acoustic instruments, not so much electric, but it can work). A dead room is the opposite, and probably the easier option. All a dead room does is it tries to cancel out the reflective properties of the room, so the sound becomes easily editable. This can be easily achieved through placing absorbent materials on your walls, such as duvets, thick blankets, pillows, but also with material such as sound absorption mats (I call it nipple foam) which are a bit more costly but are designed for this purpose. A cheap set of mats is around 20.
The same can be done for reflective rooms: Pieces of wooden boards, cheap stuff like MDF and chipboard can work well when sandwiched together, I've even heard of sheets of tin foil being used, but I can't really comment on its effectiveness.
The key is being able to use these materials appropriately. If you use monitors for recording, then make sure that the material is at the end of the sound path of the monitors (most usually on the opposite wall). Same situation for instruments, just position yourself/your amp towards the material, angling it slightly so that it doesn't feedback into your sound source.
Microphones are another tied-in element with your room sound and what you're recording. Microphones come in two different types: Dynamic and Condenser.
A dynamic mic is essentially a spring touching a metal wire. They tend to be cheaper than condensers, have many various uses in recording and are very robust. A good example is the Shure SM57. It's often considered the universal standard for recording electric guitar and snare drums. Most dynamic mics are broad in their frequency pickup range, and tend to go for mid-range frequencies (which is great for guitars) or bass (Shure Beta 52 or an AKG D112 are good and cheap for bass guitar and kick drums).
A condenser mic is a bit fancier: two plates charged with an electronic signal press against each other to form a current. They tend to be a lot more specialized than dynamic microphones, are a lot more fragile, and tend to be more expensive. They require phantom power whenever they're being used to record (most interfaces and all mixing desks have phantom power). Condensers are used to record sounds that require a specific frequency range to be picked up, so they are often used to record vocals, drum overheads, acoustic instruments and ambient sounds depending on what they pick up best. Think of the two types as passive/active.
Recording with a mic is simple enough: set it up where you want it, plug it in via XLR-to-XLR, and hit "record". However, getting a good mic sound is key, so it needs to work in tandem with your room. A condenser mic is, by far, the nicest to work with (for me anyway): Best used in reflective rooms, an acoustic guitar with a condenser (such as an AT2020 or SEX1) is a real treat to hear.
Condenser mics can be omni-directional, meaning they pick up sound from everywhere around them, Cardioid meaning they pick up in a heart shaped pattern in front of and at the side of them and Figure of 8 which means everything in front of and behind with some of the side is picked up. Some condenser mics can switch between all 3 and more depending on the mic. Dynamic mics don't require quite the same attention to detail to position, but are important none the less. When used on amps, bass or electric guitar, it's important to decide on where you want them to be placed, so they pick up the best of what you want. Most people go for the outer edge of a speaker cone, which picks up most of the midrange, bass and high frequencies, although other positions, like dead center in the middle of the cone, can produce interesting sound, most usually lots of mid-high and high frequencies are the ones being picked up in this area. It isn't recommended to use condenser mics for amps: They're very fragile on the inside (ex-wife and all) and the metal plates can be easily damaged from loud sound sources, especially bass. Also, for acoustic guitar, if you're unsure of where to put the mic, aim it for the 12th fret as a starting point. Again, consider mic positions in relation to the room and to where you're recording your instrument.
Another addition is SPL, or sound pressure level. The SPL is a measure of the sonic pressure given from one object to another, or put even more simply, it's how much your microphone can take. Do not mistake it for volume levels, but the SPL will go up the louder and wider a sound source is. Dynamic mics have a high SPL threshold: They can take a sonic pounding. The Shure SM57 mentioned earlier is a tough mic, you could use to hammer a nail (but please don't use it to hammer a nail!). Condensers have a lower threshold, as mentioned earlier: They're easily breakable if you're careless with volume levels. Remember that a louder a sound is, the less dynamic it has. This can be musically and sonically disastrous. A violin or acoustic guitar, for example, would not benefit from having the mic too close to the instrument. Same with an electric guitar and a dynamic mic, as the loudness levels of an amp (most engines prefer loud amps to capture the full sound an amp has) will cause clipping and unwanted distortion, so try to find a position a few inches away that won't cause these problems.
I think I've covered the basics anyway, hope it helps.