Reinforcements are additional units brought up to strengthen a force engaged in
combat. They are always fighting units;
rear-echelon units committed to the fight are known as
support. Reinforcements can be either planned and scheduled (as in most
hex-style wargames, when they arrive on predetermined turns) or
haphazard and random. Most real battles see the latter, as
commanders tend to try to maximize the utility of their available forces rather than move them into combat in
penny packets. Thus, reinforcements tend to be units they have managed to scrounge at the last minute or free up somewhere else rather than planned
infusions of new forces.
The exception to this rule (there's always one!) is that of planned reserves. A good commander will always retain uncommitted forces to deal with changes and/or surprises. A change is when the enemy decides to throw their troops left instead of right, leaving a smaller force of yours in a weaker position than you'd like; a surprise is when your flanks suddenly come under fire from a tank regiment that you had thought was, in fact, seventy kilometers away when you planned the battle. In these cases, reserves are used as reinforcements to strengthen the suddenly-undermassed units facing the change or surprise.
They are only reinforcements in a local sense, however; one of the criteria for being 'reinforcements' is that the forces in question arrive from outside your battle area (bringing your force's strength numbers up).
These are also distinct from replacements, troops and units added to existing files during combat or lulls in order to make good on combat losses and bring the receiving units closer to active strength.