Session Ales generally refer to beers that are lower in alcohol and crisp. Most beers in this category will be under 4 or 5% ABV and not too filling. These beers also tend not to be too anything.
The phrase IPA stands for India Pale Ale, and sits within the pale ale category. These beers are often heavy in ABV and quite decadent with their flavour profiles. Conversely, session ale refers to beers that are lower in ABV and better suited to hot, outdoor drinking during summer.