Accelerometers are MEMS (micro electro-mechanical devices) that measure their acceleration. They are normally 3-axis devices that return a reading for the x, y and z axis accelerations.
See the IMU page for monolithic components which include accelerometers with other MEMS sensors.
Like gyroscopes and magnetometers, accelerometers have to be orientated precisely on a PCB to return accurate readings. Luckily, most SMD packages used these days for accelerometers (such as the QFN and LGA package) have nice self-aligning properties during the reflow soldering process (due to the surface tension of the solder paste)..