Wood should not touch concrete. Concrete is a porous substance; it will absorb water and then release it into whatever it touches. If that is wood, the wood will mould and eventually rot. For this reason, your better contractors will advise against putting down hardwood directly over concrete. The hardwood floor suppliers will say there are ways to make it work, and there are, but none are ideal.
If you're going to do it anyway, here's the process. First, thoroughly level, clean and seal the concrete. Then, lay down a layer of breathable moisture barrier like Ty-Par or Tyvek. This barrier will prevent the transfer of liquid water, but will still allow air (and the water vapor it carries ) to permeate it, allowing the various strata to "breathe" and acclimate to changing temperature and humidity, which will prevent warpage. Then, lay down a layer of 3/4" plywood. This will be the base for the flooring nails, which are necessary to keep the floorboards tightly together to prevent squeaks. Secure the plywood to the slab with powder-actuated fasteners or concrete nails. Then, lay down another layer of breathable moisture barrier, and THEN lay down your floor, making sure to follow all the general guidelines for laying hardwood anywhere else (3/4" gap between edges and walls, nine flooring nails per board).
DO NOT put hardwood in a basement. Just don't. Tile it, carpet it even, but in a below-grade installation, it will simply be infeasible to ensure that the concrete will get and stay dry enough to not affect the hardwood in the long term. And if the basement floods, you have a lot of expensive flooring that has to be torn up and replaced.