When they vest, you either get (1) actual shares, (2) the cash equivalent (I think that option may only be available for publicly traded stock), or (3) at your option, retain the RSU for conversion at a later date.

Unconverted deferred vested RSUs might expire (and vested stock options definitely expire), but—unlike options—there’s almost never a reason not to convert an RSU (deferring for later conversion may make sense, but it's essentially always better to convert before expiration.)