Let's talk Collection.  Collection, when properly integrated into a horse's training, allows the horse to move in a more balanced, athletic way.  Collection is identified in three ways, by the length of the horses frame, the loading of the horse's hindquarters, and the relative height of the top of the croup to the top of the withers.  Collection is at the top of the training scale pyramid (rhythm, suppleness, contact and connection, impulsion, straightness, and collection), however, just because it's at the top of the training scale doesn't mean it's not attainable at the lower levels, it's just the degree of the collection that differs between the first level horse and the Grand Prix horse. 
     Picture your horse without a head and neck and your horse's body is built like a table - balanced on four legs, but when you thankfully add the head and neck to the proper end of the table, it tips the natural scale of balance to the horse's forehand.  He simply carries more weight on his forehand because he has a large head and neck to accommodate.  As I've mentioned before, I'm a huge fan of Jane Savoie, so I'm going to use her explanation of the changes in collection through the levels since she describes it best (thank you Jane)!  
     At training level, the horse is in horizontal balance meaning his balance is like it is in nature, with 60% of his weight on the forehand, and 40% of his weight on the hindquarters with his top line parallel to the ground.  
     At first level, the exercise requirements change to include leg yields, 10 meter circles, and lengthenings, so consequently the weight of the forehand shifts slightly to the hindquarters with about 55% of his weight on the forehand, and 45% of his weight on the hindquarters. 
     At second level, collecting exercises are introduced which include shoulder-in and haunches-in, and other exercises such as counter-canter, and simple changes (walk/canter), which all lead the horse to a weight distribution of about 50% on the forehand, and 50% on the hindquarters. 
     Starting with third level, the balance of the scale tips in the other direction, and the horse beings to exercise true collection.  Exercises such as half pass, volte (small circles), turns on the haunches, steeper counter-canter, and flying changes, require the horse to sit more on his hindquarters in order to perform the movements in balance, and consequently about 45% of his weight is on the forehand and 55% is on the haunches. 
     At fourth level and above still more weight is shifted to the hindquarters, compacting the horses frame, loading the hind legs, and moving the relative height his top line to an uphill frame, with his croup lower than his hindquarters.   
     Remember, collection is a gradual progression, just like the training scale, and when performed in balance and harmony, it's poetry in motion...at all levels.  :) 
Happy Riding!