What Is a Denomination?
The word "denomination" literally means "name". A denomination is just a name for a set of beliefs within a religion or for the followers thereof. Being a denomination does not mean that something is true or false or divisive. It's simply a name people use for a set of beliefs. Other religions like Shintō and Judaism have their own denominations, but this site focuses on Christian denominations, which is the most common usage of the term.
Christian's sort themselves into denominations so that they can fellowship, learn, worship, and grow with like-minded believers. Assigning the name to their beliefs makes everything much less confusing. If you move into a new town, you can at a glance identify the beliefs, theology, practices, and organizational structure of most churches without having to visit every one.
Each denomination has a unique history leading to its modern state, as well as their own reasons for believing as they do. Denominations can be fluid and change over time, as they merge, split, or shift beliefs over time due to theological, political, or social reasons. They will each likely have their own culture and nuances that may seem falling to each other, but they all still claim to be Christian.
Some denominations are essentially subsets of others. These "subdenominations" comply with the beliefs of another denomination and are considered orthodox thereby but may have a historical coming to your graphical, or mission-related reason for their own organization within the organization. Sometimes they may have beliefs that are not considered wrong by the larger denomination, but which are not prescribed by it either. The larger denomination gives people the freedom to subscribe to various beliefs, and the people in them then group themselves into smaller subdenominations along each option. People in these subdenominations are therefore members of both denominations which are not actually in conflict with each other.
Indeed, even though denominations all believe different things, they all claim to be Christian, and almost all of them acknowledge each other as fellow Christians as they share the same core principles of the faith: believing in one triune God Yahweh, who sent His Son Jesus to die on the cross to save us from our sins if we trust in Him.