Resistivity

Resistivity is a property of a marterial that stays constant no matter how much of the material you have. Think of other properties like hardness or color. For example, copper is red and soft no matter whether you have an ounce of it or a ton. Likewise, copper always has the same resistivity. A material is given a numerical resistivity value, relative to other materials. The lower the resistivity, the more conductive a material is. Copper's resistivity is 0.000000017. Iron's is 0.0000001. Silver, the most conductive common metal, is 0.000000016.

Resistance refers to how a specific piece of that material conducts electricity. It depends on the resistivity of the material, and on the size of the piece you're dealing with. For example, if you've got two pieces of copper wire of the same thickness, they both have the same resisitivity (because they're both copper), but the longer wire has greater resistance (because the electrons have to travel through more material. The exact formula relating resistivity and resistance in a wire is this:

resistance  = resistivity x length x cross-sectional area 

Don't memorize that. The thing to take away from it is that all samples of the same material will conduct as well as one another (i.e. have the same resistivity), but larger pieces will have greater resistance.