Muskets are some of the most common weapons in Empire: Total War. Used by line infantry, some light infantry, and marines on ships, they usually have a range of 70 (90 for skirmishers) and are relatively inaccurate.
Compared to rifles, muskets have a much shorter range and are much less accurate. However, they are available at a much earlier stage of the game, and they can be reloaded much more quickly than rifles. This reloading speed is usually able to trump the increased accuracy of rifles, and thus musket-wielding troops can inflict heavier damage in a shorter amount of time, provided that they survive closing the gap relatively unscathed.
A musket is one of the earliest hand-held gunpowder based weapons ever invented. Whilst by today's standards they are inaccurate, unreliable, and have a very low rate of fire (a modern compound bow would likely outmatch them in almost any situation), at the time these were powerful weapons which could cause more death, faster than nearly any other form of infantry had ever been able to accomplish. They were also nearly incapable of being used for any form of stealth however, as the amount of sound from a musket was enormous and if the targets had any questions as to where the shot came from they left a large cloud of smoke after each shot. Until rifling was invented, most muskets would be loaded with a simple iron ball-bearing (of very slightly varying sizes) and the barrel would be slightly too large for these shots, meaning that they would rattle about whilst being fired out of the barrel, greatly reducing the accuracy and reliability of the weapon.
(Picture taken from www.icollector.com)