When a boy leaves a Bear den to enter a Webelos den, he is entering a new part of Cub Scouting. His uniform will change, he will face different advancement requirements, and he will have new experiences with camping.
This is where the boy transitions from being led by the den leader to taking on the challenges with the leader helping to guide. He is preparing for Boy Scouts in which he will become the leader.
Webelos Workbook (ZIP)
If you are looking for a handbook here is electronic version.
If you are looking for a Webelos Scout Uniform Sheet