Because it's completely and totally wrong. I work for Justin.tv, so we have our own traffic graphs. And we also have ustream's traffic graphs, because they use QuantCast. And neither of our charts have anything to do with what Alexa says. They're not even wrong in the same ways.
Alexa is only correct within an order of magnitude, at best.
On the other hand, since I have the actual traffic graphs, I can say with confidence we're much bigger than ustream is. So Alexa is accidentally right.
Its the first major round of financing from a Venture Capitalist. Typically, it is after seed money, but you could go straight to a Series A without prior investments. Subsequent VC rounds count down the alphabet: Series B, Series C, etc..
Roughly, yes, but sometimes VCs do smaller seed deals that don't count as series As.
Though there's no precise definition, a series A round steps up things up a level in seriousness. It's usually quite a large amount of money (at least $500k, usually millions); the paperwork is a huge binder instead of a few sheets of paper stapled together; the terms are fairly draconian; and the investors get at least one and possibly two board seats.
