Most used ones:
ch - chain
sl st - slip stitch
dc - double crochet (US sc)
tr - treble (US dc)
htr - half treble (US hdc - half double crochet)
dtr - double treble (US trc - triple crochet)
Others:
alt - alternate
beg - beginning
blo - back loops only
cl - cluster
dc2tog - double crochet together
dec - decrease
dtr2tog - double treble 2 sts together
dtr3tog - double treble 3 sts togehter
rep - repeat
RS - right side
tr2tog - treble 2 together
trtr - triple treble
WS - wrong side
yo - yarn over hook
yrh - yarn round hook
