They are both obviously FPS games but they do have some major differences.
- Battlefield has vehicles and MANY of them. You can drive tanks, motorbikes, helicopters and more. Call of Duty doesn't have any vehicles in any of their games.
- Battlefield's maps are absolutely insanely big. Some maps are, for my taste, even too big. Call of Duty tends to have a bit smaller maps that enable close quarter combat and more shooting/action.
- Battlefield's weapons are harder to control because they mimic the real life. If you ever try shooting with a sniper rifle in BF compared to Call of Duty you will notice a big difference. The bullet will have a ''real life'' gravity drop.
- Battlefield's servers have more allowed players than Call of Duty.
There's probably plenty more but that's the most obvious ones