Hard drives come in two different standards SATA and PATA, the latter also known as IDE.
S is serial and P is parallel and this indicates how data is transferred to the disc.
SATA is the later technology and generally gives high data transfer speeds.
In External drives the use indicates which standard the hard drive inside uses. If you see eSATA then this is a newer connectivity that allows external hard drives to be connected to PCs with higher transfer speeds. You need aen eSATA connector on the PC to use this though.
If you see this on an External Hard drive caddy then it indicates the type of disc that can be installed in the caddy, either a SATA or IDE.
Hope that helps.