Lots of things! Note - if you have turned off the visual rich editor, you have a different list of icons that do essentially the same things.

b - highlighting words and then clicking this button will put them in bold
i - highlighting words and then clicking this button will put them in italics
ABC with strike through - highlighting words and then clicking this button will draw a line through them
unordered list - highlighting words and then clicking this button will put them into a non-numbered list and indent them
ordered list - highlighting words and then clicking this button will put them into a numbered list and indent them
indent with arrow - highlighting words and then clicking this button will off-set them with lines above and below, indicating a quote
align left - this button aligns your writing to the left (normal)
align center - this button aligns your writing in the center
align right - this aligns your writing to the right
chain - this links up a URL to highlighted text
broken chain - this takes away the URL from linked up text
img picture of tree - enter in the URL of an uploaded image
arrow pointing left - undo last action
arrow pointing right - re-do last action (usually used after a series of undos)
html - if you know html coding, play around with this. You can also uncheck 'use visual editor' under your profile.
? - quick link to default help section