Humanoid is a term that describes anything with a largely human shape; the word can be used as a noun or an adjective. A humanoid is bilaterally symmetrical with two arms for manipulating objects and two legs for locomotion. These connect to a central torso with a head atop (when the humanoid is standing upright). A humanoid will typically have two eyes, two ears, a nose, and a mouth on the front of the head.
Humanoids in Science Fiction
The vast majority of alien species and artificial beings in science fiction are humanoids. It is likely that authors choose to write about humanoids because both they and their readers can more easily relate to beings much like us.
In science fiction movies and television, humanoid species and androids are far easier to depict simply because human actors can portray them. There is also the matter that people can be sexually attracted to a sufficiently anthropomorphic alien.
Humanoids in Star Trek
In Star Trek, most sentient species are humanoids because an ancient humanoid race seeded planets all over the galaxy with genetic programming designed to cause humanoid races to evolve. This makes no scientific sense, but it is the reality of the Star Trek setting.
Presumably because of this, most Star Trek humanoids (usually aliens of the week) resemble humans, with added "lumpy bits" on the forehead. Some even look exactly like humans, although these may in fact be former Human colonies that never joined the Federation or later seceded. Regular aliens such as the Cardassians and Jem'Hadar tend to look more distinctly different from Humans. However, most humanoid aliens are capable of cross-breeding with other humanoid species, even though such a concept is biologically ridiculous.