Broker:

Middle man during a transaction, ties seller to buyer.

Squatter:

Someone who knowingly infringes on another entity's legal rights hoping to sell the domain name to them at a premium lower than what it would cost to take the domain by force through legal action.

I guess with domains, "squatter" is one who buy lots of them for cheap without knowing which (if any) will turn out to be useful, and "broker" is one who buys few expensive domains intending to resell them, like real estate.