From Wikipedia, SOA is an architecture, well, web service is protocol suite, and a way to communicate different application over the HTTP.
but in the real world, it seems that the SOA is only can be implemented by using web service, is it?
No, according to wikipedia SOA technologies include;
SOA is a design pattern much like OOP.
SOA is not 'web centric' in that is a pattern specifically for 'web services', SOA is simply that, a design architecture that takes 'services' into account instead of 'objects'. A 'service' (in SOA) is just a 'thing'; this 'thing' can be anything from a member function of a class, to a complete working application 10k+ lines strong, a 'service' can even be considered a 'group of services' (much like a web service acts).
As with any 'design principle' don't get hung up on semantics; SOA is not an 'end all', it's a buzz word that got over used and is oft misused (as with all buzz-words in the tech industry). That's not to say SOA is bad (no more so than OOP or a procedural pattern), it's just a tool to use in your repertoire.
Hope that can help.
Nope, web is how a service is requested, partitioning functionality so each unit of work can be requested as a service is what SOA is about. One of the reasons why a lot of implementations have gone wrong is the idea that putting a web front end on an application, makes it service oriented.