If you ask a contractor if a concrete slab will crack his response will be, "The question is not if the concrete will crack, but when and how much." Concrete invariably wants to crack. Most of the time an expansion joint is added to the floor which forces the crack in a straight line. This can be accomplished with either a line of felt, which is seen from the surface, or an imbedded strip of plastic. If you use the plastic then the crack is visible but the plastic is not.
If these expansion joints are not installed then you can very easily get cracks going every which way. This is not good at all for the longevity of the concrete slab. If you have this spider web cracking you need to seal them up before you have even bigger problems. Over the years moisture can collect in the crack and then if it freezes the force can actually break out even more concrete until you have holes all over your garage. The best way to protect from this is to use a garage floor sealer on the cracks. The process is not very technical but it does take some time and muscle power.
The first thing I always do when sealing cracks is to use a high pressure water cleaner to blow out all the dirt and concrete chips that have accumulated in the crack. By applying the stream of water at an angle you can cause most of the foreign matter to fly right out. Next, take a strong screwdriver and dig out any loose concrete that didn't get blown out. Often time you need to pry a little to get these pieces loose but if you don't get them now you will be sorry later when they loosen up. Follow this step with the high pressure water again as you will invariably have loosened more dirt and grit.
Before applying the epoxy crack sealer be sure to dry the area completely or it will not adhere to the sides. You can do this by using a floor heater or applying a propane torch. Either way, the crack must be completely dry.
Lastly, insert the sealer. I like to buy it in caulk tubes because it is so easy to spread in the crack that way. Once the crack is filled used a putty knife to smooth it out and feather the edges. It usually takes about 24 hours to fully cure so be careful to keep all moisture away during this time.