Spacer is a general term used to describe someone who lives and works in outer space, and is used much in the same way as "sailor" is when describing crewmen of a "wet" Navy ship. In formal space-going organizations, it is also a rank akin to a Navy Seaman.
Spacers come in a wide range of types and professions. They can be people who grew-up on-planet but took to a life in space, or they may have spent their entire lives in space. They can be tied to a military or corporate fleet, or free agents who drift from one ship to the next.
Most spacers are Technicians, with the rest being Space Marines or Biosocial Specialists. When people think of "spacers" they tend to think of the free agent types. As transients, they have a mixed reputation.
People who live planet-side view them as misfits - without any sense of loyalty, prone to drunken brawling and debauchery, never interested in settling down, and a bad influence on children. In their view, life in space is dangerous and does not offer the economic stability of corporate employment, nor the dream of climbing the corporate ladder. Not all free agent spacers live up that reputation, but many do their best to indulge in it.
On the other hand, spacers have something of a "cowboy" persona that draws the young, the free-spirited, the foolhardy and (in a number of cases) outcasts and outlaws. This is largely due to media portrayals of historical and fictional spacers.
Spacers are a breed apart from those who seek the safety of planetary and/or corporate life. They include countless famous and legendary explorers, entrepreneurs and war heroes who help shaped the Frontier.