Thrust is (gas) massflow multiplied by speed.
We know about the 'bucket' or 'target' type reversers, the cascade reversers and the 'petal' type reversers.
target type reversers were mainly used on single flow and low by-pass engines reversing the complete gasflow of the engine ( military engines, B707, DC 8, Fokker 28 ..) at the exhaust.
Cascade reversers reverse the fan duct flow only. The fan flow being blocked by blocker doors moving into the stream with the cascade cover sleeve moving back. The cascades are rows of vanes directing the flow to the outside front to create reverse thrust.
Petal type reversers are further simplyfied blocker doors which block the fan duct and at the same time direct the fan duct air flow to the outside front to create reverse thrust (A320/CFM56-5, A330/RR Trent).
Thrust reverser efficiency is about 30 to 40 per cent, that means for a given gas flow, they produce in reverse about 30 to 40 per cent of the same gas flow's direct thrust.