Http 502 and 503 errors

Question

Hello everyone,

I am writing a simple client which downloads Pages from Http Web site. Sometimes I received errors like 502 or 503 (Http error code), and I am wondering,

- What do they mean?

- Are they pure server side issue? Or I could improve by adding some additional information to my Http request (e.g. provide user name and password, provide locale information?)

thanks in advance,
George