The ARM chips are quite popular and being used in smartphones and tablets. Qualcomm is one of these chip manufacturers who build the Snapdragon chip which is quite popular. They have decided to re-brand their Snapdragon chips with an easy to remember names S1, S2, S3 and S4.
First Generation – S1 – 65 nm technology and features the Adreno 200 GPU. This chip can be found in the first wave of Windows Phone 7 handsets and the HTC Droid Incredible.
Second Generation – S2 – 45 nm technology and features the Adreno 205 GU. This chip can be found in devices like the HTC thunderbolt, LG Revoltion, T-Mobile G2 and Sony Ericsson’s newer Xperia models.
Third Generation – S3 – 45 nm technology with dual core design and features the Adreno 220 GPU. The chip can be found in the HTC Sensation, HTC Evo 3D and the HP TouchPad.
Forth Generation – S4 – This is the future of the chipset which will feature multi cores (dual and quad) with speeds from 1.6GHz to 2.5GHz
source - Qualcomm