Usually some kind of finders fee, e.g. $10-20k for a software engineer placement in the SF Bay Area. Often paid after a specific period of work has elapsed (e.g. 3 months). Sometimes a percentage bonus based on the first year's salary.

(I am not a recruiter; I'm on the other side of the same spam, sadly. However, my partner has worked closely with recruiters and gave me a massive braindump of the space just a few days ago. Would love to hear from some actual recruiters.)