A lover is someone who you will share romantic feelings with. This is the person that you may want to share most of your time with because you just feel a sort of high when you are together. There are some things that you feel that you can only share with your lover. When you talk to each other, there is a chance that you will talk about your relationship and how you can improve your relationship further with each other.
When you say friend, this is someone that you may also share a lot of things with. The things that you might find problematic about your lover may be known by your friend. You and your friend will usually not talk about your friendship. You just spend time together and go with the flow.