A-frame easels (also called Lyre easels) are triangular shaped easels with 3 legs – 2 in front and 1 in back.
A-frame easels can generally hold canvas up 38" by 75". They are not incredibly sturdy but work well if you are not working on a large scale. (this is the type of easel I use on a regular basis)
H-Frame easels get their name because they are shaped like the letter "H". These are generally sturdier than A-frame easels. But they take up much more space and are not as easy to fold up and store. These are great for large canvases.
There are also tabletop easels, which, as the name describes are meant to be placed onto of a table. They are small, lightweight and work best for smaller scale.
These are not all of the types of easels, there are giant easels, convertible easels, air easels and more. These just happen to be the type I use the most.