United doesn't have a "one rule fits all" for certificates, so you need to check the rules for your specific one. They are usually provided in the email about the eCertificate or printed on the back of a paper certificate. United unfortunately does not provide terms on their website for certs.
The certificates in general are restricted to airfare only, can't be applied towards extra fees like baggage or enhanced seats. But in years past (it has been a while since I had a UA cert) you could apply it to someone else's airfare if they were traveling with you on the same flights.