Narrow cracks are best protect by nuts on wire (commonly known as Wires). A minimum for beginners would be a full set from 1 - 10. On short gritstone and granite outcrops this may be enough when combined with wide crack protection. Elsewhere, most leaders would double up on wires or at least on the smaller sizes. Most climbers carry wires of a similar size on Karabiners in lots of 5 - 10 wires. They place the wire in the crack and then use a quickdraw to link it to the rope.
The best known, best value and probably the best designed are Wild Country Rocks. Having bought one set of these it is worth buying a different type of wire for your second set as they will offer slightly different placement possibilities. DMM Wallnuts are a very good alternative.
As you progress up the grades you will notice that the cracks you encounter tend to get smaller. It is at this point that investing in some Microwires will start to seem a good idea.
Wide crack protection means either carrying large Nuts (aka Chocks or Hexes) or Camming Devices, or more likely both. Whilst cams have many advantages over nuts, nuts do in fact have some advantages over cams: they are lighter and considerably cheaper, they don't walk into cracks, and they will protect a crack that gets wider inside where a cam will be next to useless. They can also be hammered into an icy crack in winter.