I'm assuming you are talking about VDB vouchers (many threads on these).

1) It is 99% of the time a free roundtrip with restrictions (14 day advance, S,T,W booking, etc.)

2) It expires 1 year from date of issue

3) It is not upgradeable

4) It earns you no miles

It is possible to get $$ instead of a free ticket voucher which are usually earned on intl tickets. These can be used like cash when buying addtl UA tickets which will be upgradeable and earn you miles.