A "tagline song" is a song form where every verse within the song contains one identical line, often called the tagline or tag. The tag can be placed anywhere within the verse, though it is most commonly either the first line of the verse (e.g., "Somewhere Over The Rainbow", "Oklahoma", etc.) or the last line of the verse (e.g., "The Lady is a Tramp", "New York, New York", etc.). It is no coincidence that the tag is often the title of the song, as it is not only repeated often, but it is generally the "thesis statement" of the song.
When improvising tagline songs, it may be easier for the singer to initiate the tag as the last line of the verse, so she isn't "trapped" into having the first lyric out of her mouth be the repeating material/tag/title of song. It is also generally easier for accompanists to recognize the song as a tagline song if the tag is the last line.
Tagline songs will usually contain at least one bridge, and the tag is almost never within the bridge.
Tagline songs are a contrasting form from verse/chorus songs, which have an entire section of lyrics that serve as repeating material (the chorus). Tagline songs only have the tag itself as the repeating material.