Spatter is made up of little bits of metal that are sent flying away from your welding area by your welding arc. Excessive spatter can result in low-quality stick and mig welds, make your welding area messy, and cause visibility problems (especially when sparks and smoke are involved).
You can't really avoid spatter completely, and it's much more common in stick and mig welding than it is in tig welding. The causes of excessive spatter occurring during stick welding are different from the causes that are part of the mig welding process. Here's a quick look at the differences.
Excessive spatter when you're stick welding usually indicates your arc length is too long. Try shortening your arc length, and keep in mind the general rule for arc length during stick welding: Don't let your arc get much longer than the diameter of your electrode's metal core. If your electrode has a 1/8-inch-diameter core, keep your arc length at or near 1/8 inch. That should help keep down your spatter.
During mig welding, one of the most common causes of excessive spatter is using too much wire. If you're getting spatter everywhere during a mig weld, try slowing down your wire feed speed. If that doesn't work, the excessive spatter might be the result of arc blow, which occurs when magnetism in your base metal affects the quality of your arc. To combat arc blow, try welding toward your ground clamp. If that doesn't do the trick, switch your welding machine to alternating current.