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'.

Lieutenant Colonel, the insignia is a silver oak leaf.

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."

Here's the list of commissioned officers for the US Army:

General of the Army (5 stars), General (4 stars), Lieutenant General (3 stars), Major General (2 stars), Brigadier General (1 star), Colonel (Eagle), Lieutenant Colonel (Silver Leaf), Major (Gold Leaf), Captain (2 Silver Bars), First Lieutenant (Silver Bar), Second Lieutenant (Gold Bar).

Lieutenant Colonel.