Here is nice document http://www.netapp.com/us/media/tr-3966.pdf
Deduplication does not compress bloks, it keeps one block of data, and other similar blocks presents as link to this block.
You can run deduplication manually, or by schedule.
Compression works only with turned deduplication. If you want to use compression, you must turn on deduplication first.
Compression can be inline and postprocess:
1. Inline - Online compressing your blocks, but slowing perfomance.
2. Postprocess - Compressing blocks by schedule. eg at night or weekend.
Both of this technologies you can turn on at volume's storage efficiency tab in OnCommand System Manager.