While hairline cracks by themselves are probably too small to actually cause any damage, if you just ignore them, over time they'll get bigger. Water will get down into the crack and with freeze and thaw cycles expand it, and then you'll have a big repair job on your hands.
Clean out a hairline crack with a piece of wire or a blast of air from an air compressor or even a can of compressed air. (Compessed air is sold at computer stores to clean dust out of computer keyboards.)
Run a bed of clear silicon caulk along the length of the crack and use a putty knife to push the caulk into the crack and smooth the surface even with the concrete.
Loosely cover the caulk (to keep anyone from walking on it) for a few hours and you're done. The clear silicon will fill the crack and stop any water from getting in and will also act as a bit of a cushion for the natural movement (expansion and contraction) of the concrete itself.
Larger cracks need more than just a layer of caulk to fix them. Begin the repair by using a cold chisel and a small sledgehammer to actually make the crack bigger.
Undercut the bottom of the crack into a reverse "V" shape. The bottom should be at least 1 inch wide.
Remove all of the loose concrete fragments and dust by flushing the crack with a hose or sweeping and vacuuming the concrete pieces out of the crack.
Paint the newly expanded crack with a concrete bonding agent (available at home stores) and allow it to set until it becomes tacky. (The bonding agent will help the new concrete bond tightly with the old concrete.)
Prepare a batch of ready mix concrete patching material in a wheelbarrow or concrete mixer while the concrete bonding agent is "setting up." Follow the manufacturer's directions to ensure that you get the right consistency.
Use a wooden float to firmly pack the concrete patch material into the area being repaired and level it with the surface.
Allow the concrete to set for about 30 minutes and then use an old broom, a whisk or even a stiff paintbrush to create a slightly rough surface on the patch that will help prevent slips.
Cover the patched area with a sheet of heavy plastic or a tarp and leave it in place for a few days--just removing it once a day to spray the patch with a mist of water. This will ensure that the concrete dries slowly. If concrete dries too fast, it will crack.
Wait a week, take off the plastic sheet and admire your handiwork. No more ugly cracks in your steps!