I'm looking to create a web app where users log in and use the service. Are these usually written in Java?
You have a variety of choices but PHP, Java and .NET are top choices. If you want to do something simple, go with PHP. Java and .NET are oriented towards large corporations.
although I personally prefer ASP.NET, using ".asp" is just as common as PHP / JAVA and is also relatively simple
yeah those are the usual server side languages that people use. you could also use python(django) or ruby(rails)