About Live Sound
In order to create great live sound a variety of factors needs to be given serious consideration. In addition to having a talented engineer controlling the mixer to optimize the sound one must consider the venue's size the music being performed and if the speakers need connection to a powered mixer. These days there are more than enough live sound packages to choose from - in fact companies like Harbinger, Behringer, and Yamaha specialize in live sound systems that can accommodate everything from single acoustic performances to massive symphony orchestras.
Speakers that need power to an external (powered) mixer are called unpowered (passive) speakers. They're typically used for larger more complex shows when having separate system components is a benefit. On the other hand powered (active) PA speakers are perfect for bands who play smaller gigs because they simplify the live sound setup significantly. A quality set of stage monitors is also something to think about; monitors point towards the stage so a performer can hear themselves better. Stage monitors are also available as powered and unpowered and they're essential for large venue concerts. Sometimes feedback can be a problem with monitors but the addition of a signal processor like reverb or delay usually fixes any issues. Lastly there's thesubwoofer: by reproducing the lowest bass frequencies subwoofers are ideal for dance music and bass-heavy groups. Subwoofers are not necessarily needed for low-volume performers and gigs in smaller rooms but they can certainly make any live performance application sound fuller and more vibrant.