There's nothing better than an easel for keeping a painting in place while you're working on it. Working vertically also means you're working on the same plane where the painting will hang. This reduces the risk of spilling anything on the painting and prevents dust from collecting on it. You can work sitting on a stool or standing, though standing at an easel makes it easier to step back to see how the painting is progressing.
The type of easel you get depends on the kind of painting you do the most. If you like working on large-scale canvases, a table-top easel is unsuitable. Likewise, if you only ever work on a small scale, a table-top easel may be more ideal than a floor-standing easel. If you enjoy standing to paint, consider a floor-standing easel. And if you paint vigorously, you'll want a heavy easel for stability.
If you paint only with watercolors, you probably won't want an easel which will only hold your work vertically. Look for something that allows you to adjust the angle at of the canvas. Oil paintings should be held vertically or near-vertically so they collect less dust. Acrylics dry fast enough for dust not to be a real problem.
The price of easels varies, starting at inexpensive sketching and display easels and ending at large-scale studio easels. If you're only just starting out, a table-top easel is probably the best (or a sketching easel if you want to stand to paint). But if your heart is set on a floor-standing easel, don't compromise and buy something else. Save up for a bit longer to get what you really want.
If space is an issue, table-top easels are great, as they don't take up floor space and can be folded up. They're available in various forms, including ones that are scaled-down versions of floor-standing easels, tripod easels (three-legged), and designs that have storage boxes. Table-top easels won't hold large-scale paintings. The ease of painting with these easels also depends on the height of your work surface. You don't want to bend down to paint.
Studio easels are large, floor-standing easels that can accommodate big canvases. The ones with a square foot (H-frame) are more stable than the ones with three legs (two at the front, one at the back), but don't fold up as quickly for storage against a wall. Studio easels can get really huge, heavy, and expensive! Let the size of the canvas you're going to use determine your easel size.
A French easel is a three-in-one easel: a sketch box, an easel, and a canvas carrier. The sketch box holds your painting supplies and palette. The legs and canvas-holding arm collapse to make it easy to carry, and there's somewhere to attach the canvas while you're transporting everything back home. The angle at which you work can vary between vertical and horizontal.
Sketching or portable easels are lightweight easels that can be taken outdoors. Display easels are made for showing a painting and tend to be a bit flimsy. They're useful for propping up a painting that's drying, but they are not suitable for serious painting.
Variations on a table-top easel, sketchbox (or paintbox) easels have a box for storing painting supplies. The lid of the box has a lip on which to stand your canvas. If you are just starting out, consider buying one of these easels that come with paints, brushes, and other supplies, inside.