It's going to depend a little on the design of the strut, but struts in general have the spring integrated with the shock, and the shock itself is the upper pivot of the steering. Think of the top of the strut as equivalent to the upper ball joint. Because of that the strut itself has to be pretty beefy, making it expensive, and the spring has to be compressed to get it out of the old one and into the new one.
Many strut systems also require realignment after installing. VW's. for example, use the strut mount bolts as the point of camber adjustment.
The job is usually more tedious than complicated. With instructions and the correct spring compressor, and enough jacks and stands, it's probably not a hard job. You'd do well to get at least a Haynes or Chilton manual, though. Many aftermarket struts come with lousy cheesy generic instructions that are no help at all. If you're new to it, budget a few hours at least, and if you live in rust country, make sure you not only budget a little extra time for removal, but make sure the springs are not cracked. And always make sure to do one side before you disassemble the other, so you have a reference if you need it. It can seem pretty simple, but then you get a stack of little rubber bushings, snap rings and conical washers, and you'll be glad you have a whole one to look at.