The 2950 has a SAS backplane. SAS slots will accept SAS drives (with the uninterrupted connector) OR SATA drives (with the interrupted connector):
The break in the SATA drive connector lines up with a break in a SATA cable connector(s) (usually separate data/power cables), preventing you from attaching a SAS drive to a SATA cable/backplane connector.
The SAS backplane/cable connector is open, so as to allow EITHER type of drive to be inserted. SAS is "backward" compatible with SATA (backward being in quotes because it is more a "side" compatibility than really "backward").
The 2950 will take SAS or SATA. They can even BOTH be used in the system, as long as they are not in the same array. SAS disks are more expensive, because they are high-performance disks (robust enterprise-level commands, higher MTBF, and higher rotational speed). SATA disks are lower-performance drives - a sacrifice made to allow for larger storage capacities. If using a PERC controller, you should make sure the disks are enterprise-class disks if this is to house important data or services, as cheap consumer/desktop drives will not cut it.