The Sanskrit word karma means action. It was used in Brahmanism to refer to those physical and verbal acts that were considered to bring positive results to the individual performing the acts (or on behalf of whom the acts were performed by Brahmin priests).
The Buddha denounced this idea. In his teaching, that which was potent for the individual was the state of mind (cetana):
cetanāhaṃ, bhikkhave, kammaṃ vadāmi
It is cetana, monks, that I call karma.
-- AN 6.63
Either way, anyone who tells you that "Karma is what happens as a result of your actions or your choices", doesn't understand the meaning of the word. What they are referring to is vipāka (result); karma is that which brings vipāka.
Either way, your question still stands:
who/what is the keeper of such actions and choices?
This is a common question, but it mostly stems from a lack of clear understanding that karma simply means action. It isn't a thing, it doesn't last from life to life or moment to moment, and it certainly doesn't come back to reward or punish the doer.
The law of karma is simply an observable pattern or regularity that exists between ethical acts and the results that follow them. I don't know of any explanation for why the universe works this way, but that it does is readily apparent.
We don't question, for example, whether torturing others is bad for the person doing the torturing (or at least we shouldn't). Obviously doing evil corrupts one's mind, destroys one's reputation, cultivates enmity, etc. (c.p. MN 129)
The real question, I guess, is why this doesn't end at the moment of death. Of course, given that death of the individual is a concept (as is the individual), there really isn't as much trouble explaining this as one might think. One's rebirth is conditioned by one's death and the events leading up to one's death; a murderer is likely to be reborn entangled with the murdered, because murdering others involves karmically potent mind states that change one's future mind states including those involved with one's death.
Another way to ask the question is how is a record of my actions maintained and kept?
It isn't. Nothing that arises lasts more than a moment, and nothing that has ceased ever returns. Karma is simply a natural law, like the law of gravity or conservation of energy, that:
“yādisaṃ vapate bījaṃ, tādisaṃ harate phalaṃ.
kalyāṇakārī kalyāṇaṃ, pāpakārī ca pāpakaṃ.
Whatever sort of seed one plants, that is the sort of fruit one will reap;
A doer of good good, and a doer of evil evil.
-- SN 11.10