Though not an officially sanctioned body, the Cardinalate is a part of the Synod of Bishops. The Cardinalate as its name suggests accurately is made up of exclusively Cardinals. The Cardinalate functions as an advisory body to the Vicarian. Since by custom, the Vicarian's all highest appointed positions are all handled by Cardinals, the Cardinalate functions like a Cabinet or High Council.
The Cardinalate also has the unofficial function as the body of approval for the Synod of Bishop's decisions. Meaning that few things get passed without the approval of the Cardinalate, and amongst them the highest and most influential members of their council.