Woot is usually one super-low-priced item per day. Electronics, clothing, watches, bunch of different stuff. My brother got a deal on computer supplies (I know, he got a pack for me, too). Some are new, some are refurbished.
A woot-off is, every time an item sells out, they replace it with a new item instead of waiting until the next day.
You saw shirts at 4:02; as I post this, just before 5, there's an e-machines desktop computer with a dual core processor (refurb) and a 90 day manufacturer's warranty for $299.99. It doesn't seem to be selling all that fast.
BoC is Bag of Xrap (replace the "x" with a "c"). It's stuff that they didn't have enough of to even offer in a Woot-off. Usually those sell out in about a minute and a half!