What's the difference between the Xperia X10 and the Xperia X10a?
The X10 is distributed in two varieties for different regions of the world, and they're internally referred to as the X10a and the X10i by Sony Ericsson. Strictly speaking, X10 is just a general term for the hardware of either of the X10 phones, so there's no "difference" since an X10a is an X10.
As for the difference between the X10a and the X10i, you can find a good explanation on the Wikipedia page. Essentially, the X10a is sold in South America and the X10i is sold in Asia and Europe. Both phones are sold in the US and Australia, but on different carriers. For instance, in the US, AT&T carries the X10a and T-Mobile carries the X10i. Because they are targeted at different regions/carriers, they support slightly different frequency bands. The rest of the hardware is the same.