The S-shaped pull describes the path that the hands take under the water; the route sees the hand shift outward on entry, scoop round and in through the ‘catch’ phase, and then back outward past the thigh. In more recent years, some coaches have adopted the I-shaped shaped path – i.e. straight back.
Essentially, the I-stroke allows a quicker cadence or turnover with the arms – because the hands are travelling a shorter distance under the water – but this comes at the cost of lower resistance forces on the hand (to pull on/with) and generating more work through the shoulder. The S-stroke will generate more power as you can utilise more space on the palms and forearms, and use the back muscles more – but cadence will be slower as a result.
The two styles aren’t all that different, but I tend to encourage athletes to ‘feel’ the water, and the resistance against your hands and forearms, which leans toward a slightly more S-shaped pull. Over longer distances, this also prevents your shoulders from burning out. Be warned, though, exaggerating an S-shape can send you off in odd directions!