eSATA is functionally identical to SATA except it's designed for external connections rather than internal ones. The differences are mostly mechanical (longer, tougher cable with added shielding and insulation) and stricter electrical specifications (but otherwise within the SATA specification). An eSATA connector can actually plug into a SATA header and both connect to the same hardware controller. If your motherboard has an eSATA connector, it likely also has at least 1-6 additional SATA plugs internally as these have been standard on all motherboards since the mid 2000s. In all likelihood your main hard drive and optical drive are already connected over SATA.
If you are not certain, you can check yourself. Take a look at http://en.wikipedia.org/wiki/SATA
so you know what to look for