At the most basic level, I'd say it's an interest, as WingedZephyr said. Generally, people join the furdom for a mutual interest in animals and, more specifically, anthropomorphic animals. It's a nice place to meet some people of a potentially more like-minded outlook on animals, and humans' connection with them, than you might find in a less specialized group of people. And ultimately, it's about meeting new people and having fun!
Some people like to express their interest through drawing or photography, some through more craft-like art forms, and others just admire the artwork of others. In the end, it's up to you to decide how to best express yourself.
