Can anyone tell me what the difference is?

From what I know, Alternate is just up,down,updown.etc. and Tremelo is the same thing?

And what speed is considered Tremelo?
I dont think there is a specific speed standard set... tremolo picking is really really fast alternate picking.
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Yeah that's pretty much it.
Alternate picking is used to actually play, whereas tremolo picking is just like "look how fast I can do this!"
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Alternate picking=You alternate the direction of your picking strokes after each note. Up, down, up, down, etc..

Tremolo picking=You play a note as rapidly as you can for the duration of the note.

In notation, there are two tremolo picking "signs". One is a slash through the stem of a note. This means to tremolo pick the note. Then there is a time where tremolo picking is indicated by two slashes through the stem of the note. This means to tremolo pick faster than if there was only one slash through the stem of the note. But still, there is no set speed.

...whereas tremolo picking is just like "look how fast I can do this!"

Well, actually. It is sometimes used to add the effect of playing one long sustained note. Although it also is used for the purpose you have mentioned sometimes
Quote by SimonLeung
So if i were to try to play at 200bpm 4 notes per beat, do I "NEED" to tremelo pick?

The mechanics of tremlo picking and alternate picking pretty similar, but the when you use them is what makes it difference. In my opinion tremelo picking is more for when you dont care about staying in time and just want to play something really fast. When you alternate pick you are picking in time.
But i do use tremelo picking "in time" to the metronome at 200 bpm for chromatic scale practice.. or it doesnt count as alternate picking?
Tremelo picked notes are usually gonna be like, 32nd or 64th notes, so I'd recommend playing with a metrenome.

Just practice going up and down one string or across two strings with it, use any scale you like, as long as it's in time.
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Tremolo picking and alternate picking are NOT the same thing.

Alternate picking - the incorporations of both downstrokes and UPSTROKES to play faster rhythms.

Tremolo Picking - a method used by guitarists (or any string instrument) to cheat the fact that the guitar has very, very bad sustain. It's when you pick the same note at a ridiculously fast pace (the faster the better -- the point is that each picking of the note is nearly indistinguishable) for a long time so that it remains audible at the end of its duration (for example, if you had to play a double whole note at 60BPM, it doesn't matter how good your guitar is, you're going to lose volume --- this is when tremelo picking is used).

The difference is that tremelo picking is always one note and usually faster (if you can alternate pick 64th notes good on you).
Yep, alternate picking would usually be when you are playing multiple notes. Tremolo picking is one note at a time.
tremolo picking is just a specific form of alternate picking . in solo playing it's used alot for effect. kind of like pull offs or vibrato , it grabs your attention and creates variety.

rapid alternate picking of one note , although there may be more than one tremolo picked note in a specific section of music.