Are there any differences between them.
I know that Steam sells the games at full price whereas Origin sells the games at a low price.
Is there a reason for this?
Errr.... It's not that simple, they are all essentially just platforms for DRM controlled games. Steam is owned by valve for example and seems to be the most popular. Origin is owned by EA and Uplay by Ubisoft.
Steam (and the others presumably) has a sale on at the moment so many games are discounted from full price.
They are different from the likes of greenmangaming for example who are essentially like the high street store 'game' who just sell video games (although I do think greenmangaming has its own platform or similar too just to confuse things)