You're probably wondering exactly what a 'furry' is, so...
What is a Furry?
You can find a good answer to that question at Fur Central. Basically, a fur1 is an anthropomorphic animal character. What does that mean? Well, it describes an animal with human characteristics, for example humanoid shape, human intellect, speech, wearing clothes, etc. This includes animals such as those in Watership Down or Bambi, humanoid animals such as the mice in Brian Jacques' Redwall series of books, cartoon or 'funny animals' such as Bugs Bunny or Mickey Mouse, animal-like aliens such as Ewoks, and mythical creatures such as centaurs and dragons.
Does a Furry Have to Have Fur?
No! A furry is any animal with human characteristics. This can include mammals, birds, fish, reptiles, insects, anything. It can even be a cross between two different species. For example, a fox-wolf, or a cat-dog. The term 'Fur' or 'Furry' can also mean a human who is a fan of anthropomorphic animals. So, a Furry can be a person who is a fan of Furries. Confused yet? You can learn more Furry terms at the Furry Glossary.
Many Furs adopt a furry character that they use in online chat, MUCKs ('Multi User Chat Kingdom', an online world where users can chat with one another) and sometimes in real life as well. Your furry is someone to draw pictures of, to write stories about, and to play as in online games such as the FurryMUCK. It is just a general alter-ego which happens to be furry. Some Furs feel a deep connection to their character, sometimes to the point of regarding it as their totem animal or familiar, a source of spiritual inspiration. The species of your furry character can be anything you want. Some examples include foxes, wolves, dogs, cats, mice, rabbits, otters, centaurs, dragons, dolphins, sharks, tigers and plenty of others too. However, it is classier to refer to them using their Latin adjectives, for example, a fox character as your 'vulpine morph'. Or lupine, canine, feline, equine...
What Do Furs Do?
Well, a lot of furs like drawing or writing about their character. A lot of them also enjoy going on various online Role Playing games. Occasionally some lucky furs go to a ConFURance. This is basically a furry convention where lots of furs go to meet other furs, artists go to sell their work, etc. They are held, usually at a hotel or conference centre, in many different countries. In some places (like New Zealand) where there aren't really enough furries to have a proper confurance, housecons are held instead. Some furs collect plushies (soft toys). Some occasionally dress up in a fursuit. What's a fursuit? It's a suit that makes you look like a furry. Some furs spend a lot of time and effort on their fursuit, and the results can look quite good.
What Does Yiff Mean?
Legend has it that 'yiff' is the sound a male fox would make when he is, uh, 'yiffing' a particularly fine vixen. Yiff also refers to yiffy artwork - that is, 'adult' furry art. And it can be used to describe moods, as in 'feelin' yiffy'. You should be aware that, while some furs do enjoy yiffy art, it is by no means compulsory. Furries and furry related art can be childish, adult, or anything in between.
Where Can I Learn More?
There's a whole heap of furry related web sites out there. Here's a few links to get you started:
Yerf - A large archive of non-yiffy furry artwork.
VCL - The VCL archives offer a huge amount of furry art.
The Furry Music Foundation - A place where some furry musicians put their works.
The MUD Connector - a good place to find heaps of places to roleplay furries.
Furry.org - the homepage of the FurryMUCK, probably the largest Furry roleplaying server there is.
FurNation - A host of many Furry homepages and artwork.
Furry Grand Central - 'The net's largest furry information resource!'
Transformation Story Archive - A large collection of stories about people transforming into other things (furries included).