They are quotation signs, have you never sent an email before everyone started using web-based emails?
They are also used as quotation signs on 2chan/futaba channel/4chan/7chan/etc. Anything after the greater than-sign becomes green (acting as a highlight for quoting other posters). As a community grows, the more it accepts and becomes watered down and generally unfunny. I'd type it all out, but as I am busy, here's some basic reading if you're interested:
http://knowyourmeme.com/memes/implyi...g-implications
Also: