Hash is a more concentrated derivative of the cannabis plant with a higher, sometimes significantly higher cannabinoid content and little plant matter. The strength varies with the part of the plant and method used to make hash.
Originally hash was considered to be the dried and pressed resin glands of the cannabis plant. More modern methods of hash making include soaking the buds or trim in ISO/ethyl alcohol, draining and evaporating the solvent off to result in a sticky blackish-brown substance that can be pressed into a slab. There is also honey oil, usually made by releasing butane through a marijuana-filled pipe and collecting the golden, highly concentrated actives that drip out the bottom.
A newer and increasingly popular form of hash is bubble hash, which is made by soaking the fresh trimmings of the plant in ice cold water and strainging it through a variable micron filter.

Don't get much hash in my area though.