Muscle Fibers

"In summary, the 7 human muscle fiber types, as identified by myosin ATPase histochemical staining are (from slowest to fastest): types I, IC, IIC, IIAC, IIA, IIAB, and IIB (Fig. 2).1,3,5"(1)

FG: Fast Twitch Glycolytic (anaerobic)
FOG: Fast Twitch Oxidative
SO: Slow Twitch Oxidative
So from the picture, type I muscle fibers use SO and the type II fibers use FOG and FG interchangeably, as the type I muscle fibers are slow and type II are fast.  Type IIB uses primarily FG while IIA and IIAC use FOG.

Motor Units - group of fibers controlled by the same motor neuron, usually all fibers of the unit have similar characteristics.  These groups are classified as such:
S:Slow Twitch
F:Fast Twitch
F Subclasses
FR:Fast Twitch Fatigue Resistant
Fint: Fast Twitch Fatigue Intermediate
FF: Fast Twitch Fatigable

Muscle Fiber Plasticity

Many people have heard about distance runners having slow twitch muscles and sprinters having fast twitch muscles.  Thus, sprinters would have a higher number of type II muscle fibers whereas distance runners will have type I.  One interesting subject of this article is the ability for the type of fibers we have to change.  Type II conversion to type I only occurs in a laboratory setting.  What this means is you hit a threshold for your muscle's capacity to improve your distance running ability.  The same thing goes for sprinting, albeit in a smaller magnitude.  Type IIA and IIB fibers convert between each other but no evidence of other type conversions has been observed in a natural setting.  When deconditioning occurs (when we stop working out) muscle fibers convert in the fast direction.  Slower muscle fibers convert to faster ones.  Additionally, the enzymes that assist in aerobic oxidative metabolism, which is required for aerobic exercise, decrease.  This is why we lose our endurance so quickly when we stop training.(1)
High-intensity resistance training, such as weightlifting, not only changes the types of muscle fibers but also increases the volume of proteins within the muscle fibers.

1 Muscle Fibers