EF - Stands for "Electro-Focus." EF lenses can be used on full-frame cameras and APS-C (1.6x) crop sensor cameras.


EF-S - Stands for "Electro-Focus: Short back focus." These lenses are specifically designed for use with APS-C (1.6x) crop sensor cameras and will not work on full-frame cameras. A Canon 7D, being a crop-sensor camera, can use either EF or EF-S lenses.


L - Stands for "Luxury." It is Canon's professional top-of-the-line lens series.


IS - Stands for "Image Stabilization." If a lens has IS, it can compensate for some camera shake.


I'll let others give their suggestions as far as lenses go...