Second Degree AV block has three distinct subtypes:  Type I, Type II, and 2:1 Block.  The level of block in second degree is variable, as well - it can be in the AV node, or below the AV node.  Type I block is commonly in the AV Node, and Type II block is generally below the AV node.  Blocks below the AV node are typically more unstable and will commonly require a pacemaker.  For guidelines on pacing in heart block, please see http://www.dallasheartrhythm.com/page35/page4/page6/page6.html. 
 
In 2:1 block, there exists a distinct relationship between the atria and ventricles.  There are two p waves for every QRS, with one p wave being blocked (either in the AV node or below the AV node) and the next beat being conducted down to the ventricles.    In 2:1 block, you do not have enough information (although There are clues to suggest where the block may lie) to identify it as type I or type II block.  Put another way, in type I block, you have several beats to observe an prolonging PR interval.  In 2:1 Block, you don't have that many beats to see if that observation can be made.  Therefore, 2:1 block is put in it's own category. 
 
ECG Features:
1.  Two P waves for every QRS
2.  The P wave that conducts to the ventricle (P wave followed by QRS) will have a fixed PR interval