(covers information from several alternate timelines)
A bullet, or pellet, was a projectile propelled by a firearm. Most bullets were made from a type of metal, such as lead. Gunpowder was used to propel the bullet by igniting the gunpowder with a spark. Damage caused by a bullet piercing something was called a bullet hole.
When talking about their first encounter with the Borg in 2153, Malcolm Reed told Trip Tucker that his security team could also fire with holographic bullets as this would have had the same effect as the phase-pistols. (ENT: "Regeneration")
In the alternate reality created by Nero's temporal incursion, Montgomery Scott described the notion of transwarp beaming as being like "trying to hit a bullet with a smaller bullet, whilst wearing a blindfold, riding a horse." (Star Trek)
By the late 24th century, weapons that fired bullets were relatively rare. In 2375, when three Starfleet officers on board Deep Space 9 were killed with tritanium bullets, Chief Miles O'Brien pointed out that one didn't see bullets every day. (DS9: "Field of Fire").