Primary Cell: the cell, operating on the primary frequency, in which the UE either performs the initial connection establishment procedure or initiates the connection re-establishment procedure, or the cell indicated as the primary cell in the handover procedure.
Secondary Cell: a cell, operating on a secondary frequency, which may be configured once an RRC connection is established and which may be used to provide additional radio resources.
Not sure about the serving cell.
The RRC connection is only handled by one cell, the Primary serving cell, served by the Primary component carrier (DL and UL PCC). It is also on the DL PCC that the UE receives NAS information, such as security parameters. In idle mode the UE listens to system information on the DL PCC. On the UL PCC PUCCH is sent.
The other component carriers are all referred to as Secondary component carriers (DL and UL SCC), serving the Secondary serving cells.
For more detail check http://www.slideshare.net/Nidhi_Arora/carrier-aggregation-in-lteadvanced-17259749 page 5/6.
