There are 3 types of muscle; smooth, cardiac and skeletal. Both smooth and cardiac are considered involuntary, which means that continue to work whether we are conscious or not, otherwise falling asleep would be a lot less appealing! (paraphrasing Tim H.) Skeletal fibers have two types of fibers; Type I, slow twitch and O2 rich blood, and Type II, fast twitch, white, powerful and big.
There are 3 types of muscle contractions; concentric, eccentric and isometric.
Concentric contractions go against gravity or against resistance (muscle force > resistance). The muscles get shorter during concentric movement and this is also considered the “working part” of any movement. In the gym, the concentric contraction is the part where you exhale.
Eccentric contractions are the “easy” part of movement (muscle force < resistance). During eccentric movements the muscle lengthens. In the gym, you inhale during eccentric movements and it is called the “negative”.
Isometric contractions offer little to no movement at all (muscle force = resistance). In the gym you will hold an isometric contraction about 15 seconds to 30 seconds, but almost always less than 2 minutes.