A jam session is a gathering of musicians who get together to play around with music. Improvised music is commonly played at jam sessions, or musicians may have a rough theme or composition to work with which they expand upon in the course of the jam session. A jam session can range from an informal gathering of street musicians who happen to run into each other to a session convened specifically with the goal of recording and publicizing the music from the jam session.
People have been playing music together for a very long time, and it's safe to say that the jam session has been around in some form or another for thousands of years. In the 1930s, however, jam sessions reached a new height in the jazz community, with musicians playing recognized standards in front of an audience, and then getting together for a heated and experimental after hours jam session. After hours sessions often included cutting contests, in which competitors cut into each others' solos in an attempt to outdo their performances.
Jam sessions are closely associated with jazz and rock, although technically people who work in any musical genre can jam. Classical musicians, for example, may jam together to explore themes, arrangements, and new ways of looking at classical music. Composers may also dabble in jam sessions, playing with other musicians to get a feel for how their work sounds and how people respond to the work on an emotional level.
In many communities, it's not uncommon for musicians to have regular jam sessions which may be open to invited guests who can play music or listen. A set day of the week or month may be set aside for a jam session, with musicians meeting in a community center, garage, or home to play music together. The musicians in community jam sessions tend to vary, depending on who feels like going. Jams are often used as practice by members of a band, giving people a chance to work off a little steam and experiment with music in a casual setting.
Attending a jam session can be very interesting. Some bands have more formal, organized jam sessions which are more polished and performance oriented, while community jam sessions may be marked by long pauses, lively debates among the musicians, and occasional moments of discordance as the musicians fail to agree on what they are playing. However, jams do give observers a chance to see the many ways in which music can be played with, and they can be quite eye-opening for people who are not familiar with improvisational music.