Variable stars are stars that change measurably in brightness. When you see stars in the night sky, they may seem to be the same night after night. But if you watch them closely, and over time, you'll see they may change quite a bit. In fact, at some level almost all stars are "variable". Even our Sun is a variable star!
This is a big web site with a lot of stuff. The AAVSO has been monitoring variable stars since 1911 and has data dating back to the 1800's. Right now the AAVSO has about 1,400 members and observers from 52 countries contributing over a million observations per year. In short, we have a lot going on and that leads to a pretty big web site.
There are hundreds of thousands of variable stars observable from Earth. To properly study a star, one needs to follow it over a period of time. Astronomers simply don't have the resources to monitor them all. This is where you come in. Observing a variable star is relatively simple. You basically compare the brightness of the variable star with that of nearby non-variable stars and then report that brightness to the AAVSO.
The core activity of the AAVSO is to observe variable stars. However, approximately half of our members do not actively go out and observe variable stars. Instead, they contribute by applying their computers and skills in other areas (a.k.a. armchair astronomy). This is a form of citizen science. Observation is just one phase in the scientific process. There are lots of other ways to contribute scientifically.