Monotheism is the belief in a single God. The best known allegedly monotheistic religions are the Abrahamic religions: Christianity, Islam, and Judaism. Other monotheistic religions and sects include Sikhism, Zoroastrianism and some Sects of Hinduism. Various philosophical traditions have also been monotheist including Stoicism.
However, many of them have polytheist features.
The Abrahamic religions feature angels, some Christian and Muslim sects feature saints, and Zoroastrianism has angel-like entities. Angels are often believed to intervene in human affairs, and many worshippers pray to saints. Additionally, these religions feature an evil quasi-deity, Satan, complete with his own followers. These religions can thus be interpreted as having one big god and lots of little deities.
Hinduism is well-known for being polytheist, and monotheist Hindus explain the multitude of Hindu deities as aspects of a single deity. This may be called "inclusive monotheism" as opposed to Abrahamic "exclusive monotheism".
Monotheism and Christianity
It is hard to work out how "monotheistic" Christianity really is. For a start some versions appear to have three gods - the Father, the Son and the Holy Spirit. Bizarrely, for some weird Theological reason this Trinity is counted as one, with "tritheism" being an Officially Recognized Heresy.
Then some Christian communities venerate saints and especially venerate the Virgin Mary. Believers Pray to her and many churches are devoted to her, as a result she is obviously a goddess in all but name.
Finally, on the other side there is Satan who has his own band of servants and also interferes in human affairs and some people also pray to Satan.
So, while the Christian Pantheon is undoubtedly less populated than, say, the Greek one, it is hardly monotheistic. We must also remember that, in general modern polytheists think of their multiple gods as really being different embodiments of one supreme god - a concept very similar to the trinity in Catholic Christianity.