pH is defined as the measure of the strength of acid in a solution. On a pH scale, 7 is considered neutral. The lower the number on the pH scale, the more acidic. Conversely, the higher the reading on the pH scale, the more alkaline. pH is a logarithmic scale. So, for each increment there is a factor of 10. So, a reading of 5 is 10 times more acid than a reading of 6.
Total Acidity (TA) is a measure of the complete acid content in a wine. While it is more accurate, this measure is not easy to ascertain. So, Total Acidity is often measured using the next best thing, which is the measure of titratable acid. Hence, the terms, while used interchangeably, mean different things, but in practice are based upon the results of acid titration.