Read about the differences between a slope sill and a pocket sill window.
There are a couple of differences between a slope sill and a pocket sill window. Firstly, pocket sill windows are designed to surround the bottom edge of the sash when the window is closed. The advantage of this is in many cases, you will receive greater air infiltration and more glass to your window. Quite a significant disadvantage, however, is that the sill looks and acts like a gutter for your window. And, in time, it can fill with dirt and debris and create major structural issues if not properly cleaned and maintained regularly.
Sloped sill windows, on the other hand, feature a sill that flushes the dirt and debris out each time it rains. A sloped sill uses gravity to flush water out of the sill of the window. A great advantage of sloped sill windows is there is little to no maintenance involved and much less chance of structural erosion of your window frame over time.