Amplifiers go back almost a hundred years, but until the 1980s they were basically exclusive to steel and electric guitars. The modern concert scene changed that when folk and bluegrass acts moved into venues where their unamplified guitars just couldn't reach the whole crowd. Other styles of music followed suit, and today, no matter what your genre, if you play acoustic in concert you'll probably be using an acoustic guitar amp.
Unlike an amplifier for an electric guitar, an acoustic amp doesn't create the sound. Instead, the acoustic guitar amp is designed to reproduce the natural sound of the guitar as accurately as possible, working in tandem with your chosen pickups. What this means for you is that, compared to electric guitar amps, the sound will not change as much with different components. A tube-based amp will still give you a warmer sound, but the effect is a bit more subtle with an acoustic amp.
When you're picking out an acoustic combo guitar amp, make sure you're getting the power level you need. For the smallest venues and practice spaces, these amps run as small as 10W. If you're playing massive concert halls, you might go all the way up to a 300W model. Scale the power level to suit the size of the spaces you play.
The speaker configuration also deserves some attention, with larger woofers producing more pronounced bass. If you want a sound that's well-balanced and not too heavy on the low-end, consider an acoustic guitar amp with multiple smaller speakers that will give that high-end punch along with plenty of volume to back it up. Some models feature an assortment of differently-sized speakers working together to boost both the bass and treble for a full, rounded tone.
An acoustic guitar combo amp will give your guitar the boost it needs to reach the furthest seats from the stage. Not only do these amplifiers project the sound of your guitar further, they also bring it up to the level of other amplified instruments so your bandmates won't drown you out—an essential for every acoustic guitarist.