In a general sense, a hymn is any song used for a religious purpose, for example for worship, or for thanksgiving, or to tell a religious story. The Psalms in the Old Testament could be considered hymns under this definition.
More specifically, the term 'hymns' is often used to refer to a kind of song traditionally used in Anglican, British Catholic and Methodist worship. These songs were written to be sung by a large congregation of people, and generally lend themselves to being played on the pipe organs commonly found in British churches. With this in mind, they usually have a simple meter and a repeating tune with stanzas of four to eight lines. Many of these hymns were composed in the eighteenth and nineteenth centuries; Charles Wesley and Isaac Watts are two examples of prolific hymn writers from that period.
In modern worship, some practising Christians draw a distinction between hymns and 'worship songs', which are written in a more modern style and often have influences from pop music. For example, it is much more common for 'worship songs' to have a chorus or refrain that is repeated between each verse, although some traditional hymns do have this feature as well. Also, worship songs may be written for more modern, often guitar-based accompaniments, although this does not always deter church organists from attempting to play them.