Database compression is the SQL Server 2008 Enterprise ONLY features. MSSQL offers 2 type of the data compression. Row compression and Page compression.
To estimate how much space you can save by running sp_estimate_data_compression_savings. The store procedure takes 5 parameters: Schema, object name, index name, partition name, compression type( Null, Page or row)
We can check the current table compression level using below query
Obtain the table size before the compression
Change to the row compression
Table size after row compression
Change to the page compression
If you want to uncompressing the data, you can use DATA_COMPRESSION = NONE option
After the data compression is being enable, if you query the sys.dm_db_persisted_sku_features, you will see you have use this enterprise edition feature. By checking this DMV, we can verify whether we have use certain features only available on the higher edition. This is important if you need to do the migration or even just try to restore your production database which is running on the Enterprise version to your development environment which may only running the standard edition.
Reference & Future reading
Data Compression: Strategy, Capacity Planning and Best Practices http://msdn.microsoft.com/en-us/library/dd894051(v=sql.100).aspx