This the name given to a group of minerals that are made up of potassium and aluminum silicates (orthoclase), sodium and aluminum (albite), or calcium and aluminum (anorthite). This is a mouthfull, I know. The word comes from the German "feldt spat", meaning "field spar", "spar" meaning common clevable material - the material dredged up on farm lands during plowing. Most of them are not affected by acid (exception the Calcium rich plagioclase - Anorthite). Orthoclase got is name from the Greek phrase meaning "straight fracture", Microcline from a Greek phrase meaning "small incline", and Plagioclase from the Greek phrase meaning "oblique fracture".
Here are some pictures that might help you see what we are talking about.