A cumshot is the definition of both an act and an object: specifically, it is the act of the ejaculating or depositing semen on another partner's body, when arriving at a sexual climax. When it's aimed specifically for the face, it's known as a facial but a cumshot can occur anywhere else on the body.
Popularized by pornography, the ejaculation of seminal fluids on women (and, sometimes, men), is a highly anticipated and eroticized image that can, when practiced privately, straddle the edge of moving into kink or BDSM practices.
The "shot" in the term comes from its popularization and mainstream entry through pornography. Here, the scene that is being filmed always ends in a release of sexual gratification that is most visibly signaled by a cumshot on the other partner's (or partners') chest, buttocks, face or stomach.
Hardcore porn promises this externalization of gratification through this act and it's called "the money shot" because it is a selling point for the film. Even when it's not being "filmed", a cumshot now popularly references a male ejaculating over his partner's body.
Although the act of squirting semen has drawn many critics who find it aggressive and degrading, as well as violent erotic, when couples dare to try this in their own lives, it can be rather sexy and gratifying. While it's not a mandatory part of oral sex, couples who both want to try it take a more sex-positive view of the act and of each other.
It can be a great way for the receiving partner to have a chance to feel like the naughty one and unleash her (or his) inner porn star while the partner who is ejaculating experiences a visual and auditory sense of power and sensuality that can heighten his orgasm.
In this case, talking before hand and consent is a large part of keeping the fantasy alive during the act.
Like any exchange of bodily fluids, ejaculation over others' bodies can be sexy but not without its fair share of health risks. Infected semen still has the chance of permeating the skin, especially if the skin is broken from beforehand.
For obvious reasons, the receiving partners is at a higher risk of contracting not just STIs but just a general infection. The risk is heightened when it's a facial -- so get tested first and play after.
Even if both partners are clean and have been tested, different skin sensitivities can play a role in experiencing adverse effects of a cumshot.
People who have previous allergies, such as those who are prone to psoriasis or eczema are more likely to have irritated skin due to a cumshot. And this is not even due to the act itself but because washing away the fluid can cause further dryness and itchiness.