A "verse/chorus song" is a song format that has a group of lyrics (the chorus) repeat in between unique groupings of lyrics (verses). Choruses will often use lots of internal repetition of either words or entire phrases, and the title of the song is commonly found within the chorus.
Some examples of verse/chorus songs are "Let It Be", "Get Back", and "Yellow Submarine". Other groups apart from The Beatles have also done verse/chorus songs, and these songs include, "Jeremy" by Pearl Jam, "Heart Shaped Box" by Nirvana, and "Purple Rain" by Prince.
Many verse/chorus songs have bridges in them, which usually do not contain the lyric material of the chorus.
When improvising group numbers, it is common to create the chorus first, and then repeat it, so that the group can have an easier time of learning and remembering it.
When improvising solo numbers, improvisers may find it easier to initiate a verse first, instead of the chorus, to allow time and discovery to inform the choices for the actual chorus, and to avoid having to be "married" to the first words sung as the chorus of the song.