A knit-a-long is when many people get together, generally online and knit the same thing or knit different items within the same period of time or even knit similar things near the same period of time. Some knit-a-longs have a time period they are trying to cover, such as waiting for the last Harry Potter book to come out and everyone works on the same project.
At Ravelry there are always KALís going. There is a big KAL for the Winter Olympics and there are knit-a-longs for the Summer Olympics and the Tour de France, where everyone who joins in knits the same project or just something that will challenge them during a specific time period. The appeal of a knit-a-long is that much like getting together with friends to knit you can ask questions when you run into trouble, by joining the group online. While working on your project you can post pictures of it to share with the other people also working on the project. It is a great way to keep working on a project if you like the peer pressure of competition, but not competition in the sports sense. For example, if you have a hard time finishing projects, joining a knit-a-long will sometimes keep you on track as you see other people continuing to knit, long after you would have set the project down.
KALs sponsored by local yarn shops are becoming quite familiar. They often involving meeting weekly at the local shop and knitting with a group that is knitting the same thing you are. This allows you to get help, share tips, and commiserate about your project. Once again, it helps keep you motivated to work on this project so that it does not become an unfinished object languishing in your closet.
Summer is a good time to start a KAL to get a heads up on holiday gifts, meet once a week with like minded knitters and who knows what you can accomplish.
Another good reason to join a knit-a-long is to try a new technique and have a knitting group to share questions and comments with. If you have never knit socks, joining a knit-a-long about socks is a good place to start. There will be other people knitting first socks and they will have many of the same questions you will probably have. The group discussion boards will give you many ideas and, for me, the comfort of knowing that everyone has their own way of doing things and there is not one correct way of doing anything in knitting beyond the basic knit and purl stitch.
There are also knit-a-longs that follow a knitters book throughout the year, such as The Knitterís Almanac by Elizabeth Zimmermann. The group works on one project a month, the group is very flexible, some people skip months, some take more than one month to complete a project and some just chime in with tips.
Knit-a-longs are as varied as the people who knit. Knitting is often a solitary pursuit, but with knit-a-longs it is way to share the craft with other knitters.