Very basically speaking, the difference is one step in rank. While a Lt. Colonel is normally referred to a Colonel, a Colonel is sometimes to referred to as a "full bird colonel" to distinguish the difference. In the Army at least, a Lt. Colonel wears a clover leaf rank insignia (silver, I think) but a full Colonel wears an eagle insignia, hence the reference "full bird."

6 cuz he can

Lieutenant Colonel, the insignia is a silver oak leaf.

Lieutenant Colonel insignia is identified by a Silver Maple Leaf.

In the US Army it is a cluster of silver OAK leaves.

In the British army it is a crown and a single 'pip'.

Actually, none. The term is a slang term, to indicate the difference between Colonel (insignia is an eagle, or "bird") and a Lt. Colonel, insignia silver oak leaf. The Army, Marines, and Air Force all have Colonels (Navy calls their Captains) but the term is Colonel, not Bird Colonel, or Full Colonel.

On the lighter side, Col. Harlan Sanders of Kentucky Fried Chicken fame might be considered a "bird colonel".