What
Webster_1913 said, except, there's a little more
detail.
Yeah, this poem celebrates a marriage, but it has no fixed form (like a sonnet, etc.). However, there generally is several things that are common. The subject is a specific marriage; it praises the bride and groom; it mentions something about the wedding day, and give blessings for the union and wishes for happiness. Sometimes they tell about the bride and groom's past.
Because of all these nice things, they are not generally short poems. It can range from 40 to 400 lines.
Sappho is credited with making this a distinct literary form, even though these poems exists in most cultures. Back then, occasionally the poem was sung by a chorus of girls and boys right outside the honeymooners' room.
Kinky.
The first one in English was written by Sir Philip Sidney, who celebrated his own wedding in 1580.
I think this would be a bad wedding gift, personally.
Forms of Poetry