There are several kinds of easels. Its use generally depends on the size of the canvas you will paint, the place where you will work and also on your purse...
Here are some easels you can find in shops :
If you think to work at home or outside, you will need a portable easel. There are some excellent ones in aluminum.
However, a portable easel is not as practical as a fix one design to be very stable. If you have an easel and you have enough money to invest in a new one, do not hesitate to buy a fix one. But be careful, if you have to select your investments, choose to buy some very good brushes before thinking about an easel. You will manage your paintings with a less expensive easel but not with cheap brushes.