double stops. you put down two fingers at once--on two and strings frets (or possible, one finger, barred slightly, on one fret and 2 strings) . its like playing a chord, except its only playing two notes. its like playing one note, except you do it twice, simultaniously.
all the best.
(insert self-aggrandizing quote here)
^this andits pretty easy, just choose one of the notes you are using in your solo and instead of just that note grab the one on the same fret but eithr the string above or below and hit them both....

you can also look into the intro (main riff, you know the one your most likely thinking of) of smoke on the waterit's all doublestops......

also any song that you hear them in is a great song to work on them...
Quote by Dirk Gently
Some pieces are only meant to be played by people with six fingers on their fretting hand. Sorry.
they got it covered. and a power chord is a kind of double stop but guitarists don't usually call it one. the common double stop in soloing, where the 2 notes are on the same fret on 2 strings, is essentially a backwards power chord, with the 5th below the root, or u could call the low note the root and the other note the 4th. u can do all kinds of double stops tho, major thirds (for example 32xxxx), minor thirds (31xxxx), octaves (3x5xxx), 7ths (3x4xxx), minor 7ths (3x3xxx), and 6ths (3x2xxx). of course u could do dissonant stuff like tritones but u get the point.
Another fairly common use (more so in jazz) are octaves.. Two notes an octave apart. Some folks play extended solos and melodic lines using octaves.