A commitment ceremony is often very similar to many other kinds of weddings. The difference is that rather than being a legally binding ceremony, it is simply a public affirmation of a couples commitment to one another. Generally, the couple is a lesbian, gay, or transgender couple, and thus are unable to marry under the law.
A commitment ceremony may be religious or secular, formal and traditional or loose and unstructured. The makeup of the ceremony will depend on the rules of the officiant/house-of-worship and the couple's own preferences. However, generally speaking, these are the key elements:
The officiant welcomes guests to a celebration of the love and commitment between the couple. He or she will probably also say a few words about their relationship, or about marriage/commitment in general.
This is the part where the couple declares their intent to be a committed or married couple. As in any kind of wedding, they will make promises about what that commitment means. They may promise to love in sickness and in health, in richness and poverty, till death do they part. Alternatively they may write their own vows.
A religious commitment ceremony will likely incorporate hymns and scripture readings that focus on love. (Many religious officiants will have a standard set of music and readings that are often used at commitment ceremonies and weddings.) A secular ceremony will usually also include music and readings about love, including poems, passages of literature, famous quotes, personal writing, pop songs, and classic wedding music. It may be gay/lesbian/transgender focused or very general, depending on the couple's personal preference.
Exchange of Rings
The couple exchanges rings, and says a few words about what these rings mean. It may be... With this ring, I thee wed I give you this ring as an expression of my love and commitment to you I'm honored to give you this ring as a symbol of the promises I've made to you today, and a proclamation to the world of the love I have for you.
Or anything else the couple wishes to say (working with their officiant to craft it - some religions may have rules regarding the ring ceremony)
Pronouncement of Marriage
The officiant announces to the guests or congregation that the couple is now married (joined/united/wed - whatever word you prefer to say) and invites the couple to kiss. Some couples may not be used to kissing in public and thus may only have a very small kiss, or forgo this part altogether. Others will relish the moment to have the opportunity to kiss each other in front of their loved ones, proclaiming their love, and pride in having that love.
Most couples will follow the ceremony with a reception of some kind. As with all weddings, there are no rules as to what this should be - it can be very formal and traditional, or as casual as a backyard picnic. It may include traditional wedding elements such as the first dance, cake cutting, and bouquet toss, or may just be an unstructured party. Generally the invitation will give some clues as to what it will be like (e.g. Please join us after the ceremony to toast the happy couple or A reception at the Springfield Country Club will immediately follow the wedding)