First - assuming it is on-grade - i.e. built on ground level, not over a crawl space or downstairs or basement or overhanging a slope. If it is NOT on-grade, you need a structural engineer ASAP, and do not park in it - you have a structural failure in progress. I would also look into my homeowner's policy to see if structural failure is covered, and if so have an adjusted come out to look at it.
Now, for the normal case where it is on-grade - i.e. no rooms or open space under it, and you drive right out onto the driveway from the slab.
1) If minor settling (say less than 1" maximum from center to edge, IN ADDITION to any drainage slope that was already there), with narrow random cracks or relatively straight cracks towards the corners, this could be due to inadequate compaction of the fill under the slab. Unless it is a new house still under warranty, I would not worry unless it is more than hairline cracks (i.e. wider than a ball point pen tip). A normal garage floor, after 10+ years, will probably have 3-8 of these, with some along the edges, and most in the center portion of the floor, commonly radiating out from the floor drain if you have one. Can be washed clean, broken pieces dug out, and patched with garage floor repair epoxy, with portland cement mixed to soft dough consistency and trowelled in, or with concrete crack repair caulk (in a tube, like siding caulk, applied with caulk gun).
2) If cracking is aligned with the center of the slab and opening at the top (rather than spalling or splintering the concrete), then it may be settlement of the rest of the house dragging the slab down and "breaking its back". If cracks no larger than in 1) and not growing noticeably, then I would not worry - repair as above.
3) If cracking and sagging parallel with drain pipe from floor drain, then could have a broken sewer pipe. If keeps sagging more, get sewer and drain contractor to clean and run a camera in the sewer to look for broken pipe, ESPECIALLY if your main sewer pipe exits the house under the garage slab. (There will normally be a vertical about 4 inch cleanout pipe, open or rubber capped or plugged, sticking a few inches above ground level about 2-4 feet outside the foundation wall where the sewer pipe exits the house).
4) If only cracking lightly right at the abutting edge of slab from the house, probably not a problem - house and garage slabs will settle differently, with house settling more because the central part of the house sits on the house slab. This will commonly cause spalling and cracking, and sometimes pulling away, right at the edge of the garage slab.
5) If slab is generally cracking up and pieces are broken free and raising above one another, like a sidewalk breaking up, then you have a general failure of some sort - either bad construction, too heavy a vehicle for slab thickness (like large SUV or loaded pickup on 4" slab), or serious foundation movement. I would call a civil engineer specializing in residential foundations.
Search the List (in green banner bar) for local engineers and reviews. To find which local engineers are Foundation or Residential specialissts you may have to match their names agasinst specialty listings in your local yellow pages.
Feel free to answer here with more description of type, direction, extent, size etc of cracking if you want to get more detailed.