A bubble happens when people buy something (stock, real estate or even tulip bulbs!) that they know is overvalued, on the ground that some "greater fool" will arrive later in the party and buy it at a still higher price. It's a kind of Ponzi scheme; at some point, there aren't greater fools anymore and the last players lose.

What we see now may be some misguided investments, bad companies, etc. Obviously, all the crap will end up being cleared at some point. But it's not necessarily a bubble.

Probably, but this does not mean that there aren't good ideas.

Well, considering most of the buildings from the first dot-com real estate boom AND existing officers are still mostly empty, it's not as bad.