A partition controls aggregate, or total, traffic through a class, while policies control individual TCP connections.
If you apply a 100k partition to Inbound/HTTP, you are saying that all inbound HTTP traffic can use 100k -- no matter how many connections there are. If there is one connection, it can have 100k, but if there are 20 connections, they must share and they each get about 5k.
However, if you apply a 100k policy to Inbound/HTTP, you are saying that each individual HTTP connection gets 100k. A simple web request can generate several connections, each of them getting 100k.
For more information, see Partition Overview and Policy Overview in PacketGuide.