To 'keep someone informed' implies a longer period. This suggests that you will tell someone every time you receive some news about a particular subject. It's similar to 'keep you up to date' or 'keep you updated', and suggests that this is a continuous and ongoing process.
If you 'inform' someone, this is a single act of giving them one piece of information on one occasion. It's a more formal/businesslike word for 'tell'.