Rikki,

In case you don't have time to look each one up.....

Guitar riff:
a sequence of notes that usually repeats ie: the intro to The Beatles "Daytripper"

Guitar strum:
dragging a pick or finger over multiple strings of a guitar to produce a chordal sound, as opposed to picking single notes. Popular with rhythm players ... just to sing over top of.
aka: 60s folk music(Puff the magic dragon)

string pad:
A lush, sustained sound that "lays under" another sound like a mattress pad lays under your sheets. Covers up lots of empty spots and makes a smooth presentation. The feature sound is louder and more dominant than the pad. Choirs make great pads too.

Piano arpeggio:
Playing the notes of a chord in succession instead of all at once. eg: a C7 arpeggio could be C-E-G-Bb-G-E-C. They don't have to go up AND back, some go up...some go down.... it just means that you break up the chord, one note at a time. They usually repeat. (Intro to "I will survive")

piano comping:
This is just playing the chord changes, like from a lead sheet. It's not a complete arrangement ... just the background chords. You can add accents and little "fill lines" (riffs), but the melody is absent from comping. That's the soloist's job. Think of the beginning to Chicago's "Saturday in the park"

brass stabs:
Very cool way to accent a song. The brass section all hits the same chord at the same time. Usually a short, precise blast and often trails off from high to low.(fall offs)
Stabs are effective to accent a particular beat of a measure. Think of Wilson Picket's "Midnight Hour". The intro is loaded with this type of accent.