Jews who bought the right to collect Rome's taxes and kept the surplus — collaborators and profiteers at once.
The system was tax farming: a man bid for the right to collect in a district, paid Rome up front, and recouped whatever he could. Anything over the quota was his, and there was no published rate to appeal to — which is why John the Baptist's instruction to them is simply to collect no more than is appointed. The offence was double, being both extortion and collaboration, which is what puts "tax collectors and sinners" together as a stock phrase, and what makes the calling of Matthew and the dinner at Zacchaeus's house scandalous rather than merely generous.