What is Karma?

What is Karma? I have a hard time understanding this. No matter what I read or what I listen. I always feel that I have a good grasp of this and then at times it doesn't make any sense and I am back to square one. Well, my question has always been - If what we are reaping is the product of the past or due to the past 'Karma' we did. Then why don't we remember the past? so at least we know why we are going through what we are going through and we 'learn' from our past or mistakes we did so that we will not make the same mistake again?.

I know there is a better and more profound explanation to my question.. and I just need to find it. and hopefully, one of you or some of you may be able to answer my question and enlighten me :)

There are many definitions to Karma - Karma is actions, ie - Good karma is good actions and Bad karma is bad actions. Some say - Karma is simply the process of cause and effect. If you do A, B will happen. If you plant tomato seeds, and water and look after them, tomatoes will grow.

I do have other questions -

How does Karma influence our life?
Karma Vs Destiny? How do we explain this?