The term "humanoid" refers to any being whose body structure resembles that of a human. In this sense, the term indeed describes primates, as well as mythological creatures and artificial organisms (robots), especially in the context of science fiction and fantasy fiction. An android or gynoid is a humanoid robot designed to look like a specific gender, although the words are frequently perceived to be synonymous.
Usually, a fictional humanoid species has the same basic body outline as a human, being bipedal with hands which include fingers and opposable thumbs, but differs in details such as number of digits, coloring, ear form, presence of hair, average height and weight, size of nose, form of skin, "extras" such as horns, plates, claws, tails or multiple appendages, limb structure (such as having digitigrade legs) and taxonomic lineage (being descended from reptiles, fish, rodents, marsupials, or a phylum not evolved on Earth, perhaps, instead of primates). Reptilian humanoids are a common concept.
Most of the aliens in television and movies are humanoid, since it is easier for a fictional character to be a disguised human actor. However, there are various methods for presenting non-humanoid characters, for example computer graphics, creative costuming, and puppetry, as seen in shows like Farscape and the new series of Doctor Who. Many aliens and robots often call humans humanoids, although this probably has more to do with translating alien languages, as the word human would appear to be limited to Earth's population only.
Almost all biologists find it unlikely to have a universe populated by unrelated creatures that all look human. Only a small minority of biologists believe that a species would naturally drift towards bipedalism when achieving sapience as we know it (e.g. Russell's hypothetical troodon-descended sapient, the Dinosauroid).
In entertainment (particularly Science Fiction) the trope of humanoid aliens is frequently seen, and occasionally a reason for this to be the case (at least within the scope of the fictional setting in question.)
For example, the episode "The Chase" of Star Trek: The Next Generation explained the humanoid denizens of the Star Trek universe by advancing the story of a primordial humanoid civilization, the Ancient humanoids, that seeded the galaxy with genetically-engineered cells that guide evolution toward humanoid life (see panspermia). In Stargate SG-1, many if not all of the aliens encountered are human, and this is explained by them having traveled from Earth in the distant past (See Children of the Gods). In most cases, like the classic Doctor Who serials, the reason for the similarity is not explained (although The Big Finish Productions audio play Zagreus offers a more sinister explanation: that the time lords may have seeded the universe with biogenic molecules so that only intelligent species that approximated the Gallifreyan humanoid norm would develop), and it is regarded simply as a dramatic convention or artistic license, requiring suspension of disbelief.