In the US military (excepting the Navy), the fifth officer rank is lieutenant colonel. The Navy equivalent is called a commander, except when at the helm, at which point he is referred to as "Captain." Likewise, in the other three services, a lieutenant colonel is usually called simply "Colonel" or "Colonel last name," unless it is unclear which officer is being named. Since O-5 is usually the grade at which an officer receives his or her first command, the personnel serving under this command refer to him or her as "The Colonel" to distinguish him or her from any other equally-ranked (but non-commanding) personnel in the unit.
Earning the rank of lieutenant colonel usually requires an advanced degree in some field of study--business and management are popular, but technical fields are also appreciated--as well as superior performance in a subordinate command position, such as being XO for the commander of a wing or a brigade. Those officers lucky and skilled enough to be in combat arms will earn their silver oak leaf by demonstrating leadership in the field, and will typically make rank earlier than the remfs in the same service. These officers will gain an operational command, such as a fighter squadron or a fighter wing in the Air Force.
A lieutenant colonel makes an ideal vice commander to a "full bird" colonel if no command position is open. The light colonel can help run the unit with a good cop, bad cop leadership style, gain experience and respect as the good cop, and learn how to be the bad cop when the colonel retires or gets a new assignment.