A proxy, in its most general form, is a class functioning as an interface to something else. The proxy could interface to anything: a network connection, a large object in memory, a file, or some other resource that is expensive or impossible to duplicate.
Proxies are just about everywhere. Whenever you use NHibernate, WCF or Remoting – you are using proxies. In fact, proxies are such a success that they are literally baked into both the language and the platform. In .NET we have TransparentProxy and in Java has
At any rate, proxies are really useful, especially when you think about dynamic proxies. I am far less fond of static proxies, although we use them as well. Dynamic proxies are quite useful to add behavior, especially cross cutting behavior, at very little cost.
That said, one of the major issues that arises from using proxies is an inherit assumptions that the proxy is the same as its target. Commonly you see this happening with remote proxies, where it isn’t obvious that actually making the call is expensive as hell.
Proxies also tend to be used mostly for the infrastructure of your application, rather than for actual application code. In particular, business logic, rather than cross cutting concerns, is really hard to figure out / debug when you have it spread around in proxies.
Recommendation: Use it for infrastructure or cross cutting concerns. Try doing so using dynamic proxies, rather than by generating proxies by hand. Avoid putting business logic there and be aware that by using proxies you are hiding what is going on (that is pretty much the point), so doing non obvious things should be avoided.