crossovers are used to separate the different frequecies. there are 2 types of crossovers, electronic and passive.
depending on the type of setup, a crossover will generally separate the sound to bass, mid range, and high(treble). any good system will incorporate crossovers. if you need specifics, email me with any questions.
all crossovers are electronic, the two types are passive and active. Active crossovers require a power and ground connection. Passive units, which usually come with a set of components, separate the signal with no external power connection. Most amps now have built-in crossovers. You friends are most likely using passive units that came with their speakers or using the built in ones on their amps. The only other way would be to use a 'filter' which simply applys a high- or low-pass on the signal. Active crossovers are generally used in systems where each pair of speakers is powered from one amp (an amp for the tweeters, one for the mids, etc..)