Hopefully the link below works. If it does, take a good look at the simplified image of the turbine. You'll see two inner casings (one high pre4ssure, one low pressure) and a blade carrier. The inner casings are between the moving blades (high- or low-pressure section) and the turbine "shell" and act as a seal.
The blade carrier is similar, except it provides the sealing for the non-moving blades (usually the last three or four) in the low-pressure section.