The stitch can look almost identical on the front, it's the treatment of how the thread behaves on the back of the work that can define the two.
A back stitch will always have double the thread cover on the back of the work, and a line can be worked, as a continuous line of stitches, with no gaps of fabric showing on the front.
A back stitch is generally used for outlining or lettering or for doing a short distance of work outside of the cross stitched areas. Straight stitching is really good for long or diagonal areas such as animal whiskers, hair, flower stems and so on. It might take a combination of both kinds in order to get the effect you want.