Foundation cracks can either be a nightmare or nothing at all. The biggest thing to look for is, is it getting worse? When we deal with them we commonly use reference marks along the crack. Take a fairly permanent marking device and mark both ends of the crack and put two more marks on each side of the crack along its length so that if you connected them they'd form a capital I with the crack going through the center of the I. Take measurements between the top and bottom of the I and note them next to the crack, check the crack frequently at first until you're sure its stabilized.
Depending on if the crack is getting worse or not (if the measurement increases at all or the crack moves past one of the marks on the ends) you either have an ugly blemish or a major issue. If the crack is growing consult an engineer or hire a professional to make the repair. This will likely involve jacking up the house and replacing a section of the foundation. Worst case scenario would be the foundation is on a sinkhole and that will have to be stabilized at which point I'd look for a new house... if you're in an area with a lot of limestone there's a decent chance of this.
If it's stable and not growing it's not nearly as much of an issue however, if its noticeable during a walk through it will most likely scare off potential buyers if you ever go to sell the house as they won't know if its stable or not. If its stable and water is leaking through the crack it can either be filled with a thick epoxy or mortar to prevent the water leak but try to fill as much of it as possible. If it's exposed to freezing temperatures (not below the a decent bit below the ground level) you'll have to seal it from the outside as well which may require some excavation to get to it.
