ABEC
Stands for Annular Bearing Engineers' Committee. This committee works to determine standards for bearings for the Anti-Friction Bearing Manufacturers Association. The ABEC scale classifies different accuracy & tolerance ranges into classification of 3, 5 and 7. Classified by rotating accuracy and other measurements. The ABEC-3, ABEC-5, ABEC-7 ratings you see for bearings are supposed to be indications that the bearings meet the stated ABEC specifications of a certain precision level. ABEC-7's are supposed to be the fastest.

Bearings
These are those metal things inside the hub of your wheels. There are two per wheel. Inline skates currently use bearings that were already standard in the bearings industry, before inline skates were popular. They are "608" bearings, indicating the inner (6mm) and outer diameters

Bearing Spacers
These are those small parts that go in between your bearings so that the axles can go through your wheels. Most stock spacers are metal.
Anti-rocker
This is a wheel configuration used by many rail-sliders which has the larger wheels on the toe and heel positions, and the smaller wheels in the middle to allow the sliding to take place without the wheels being in the way.
Rocker
The 2 middle wheels are lower than the 2 outer wheels, either through frame spacer adjustment or using bigger sized wheels.  This is to achieve a tighter turn and is used mainly on hockey and freestyle skates. Most skates come as a ‘flat’ wheel set up and the skater changes the wheel configuration to suit their skating.

Frame
Refers to the skate frame where the wheels reside. They come in various measurements and materials (aluminum, carbon, magnesium).

Durometer
An industry hardness rating for polyurethane, which is the primary wheel material. Ratings such as 78A or 85A are usually seen on wheels. The higher the rating the harder the wheel. 100 is the highest (although no skaters probably go beyond 92 or 95).For smoother or indoor surfaces, a softer wheel is best. Soft gives more grip, while hardness gives more speed.
Wheel Diameter
Wheel diameter can range from 56 to 58mm on aggressive skates to 78 84mm on recreational skates to 90 to 100mm on fitness/marathon skates. A bigger wheel is a faster roll and cushions small bumps on the skating surface.
Frame Spacers
These are those small parts on your skates that go between your wheels and the frames. Many skates have eccentric, oval shaped frame spacers so that you can flip them 180 degrees to rocker your skates.

Soul Plates
These are the replaceable parts of the skate known as the 'soul spot'. Depending on the manufacturer, they are called sole plates, soul plates, soul frames or grind inserts.

UFS
Universal Frame System. The mounting specifications for frames and skates have been standardized. Any UFS frame can fit any UFS skate. This quality will be noted in the description if true for that skate. More and more common as the years go on.

Wheel Rotation
Polyurethane wheels eventually wear down, but you can often extend the life of your wheels by flipping and/or rotating your wheels amongst themselves so that you can skate on the less-worn areas of your wheels.