If you believe in it, then it's real. Furthermore, if it
can be real, then it is.
Philosophy aside, I would like to add onto what
@ShadedSelf said about it depending on your definition of karma. It also seems to depend on your belief system. For example if you subscribe to the Kabbalah system, then karma is rather transformed into the idea that you attract whatever energy you put out in the world. If you vibrate in a low, malevolent frequency and mean harm to others, harm shall come to you, and vice versa. I suppose that's a form of karma, but the central belief in Kabbalah is that it is your certainty that it's all for your good that opens the floodgates for good fortune to come into your life. That no matter how bad a thing happens, you must preserve your certainty that it's happening for a reason, and that reason is to make you better and stronger. So in that system the cycle goes like this:
You cause harm to someone out of insecurity that they're better than you (for example) > later in life harm comes back your way because of this > an opportunity arises for you to recognize the reason why this is happening to you > you either take the opportunity and learn from it, becoming less insecure in the process, or > you don't learn anything, you suffer an blame your bad fortune, creating fertile ground for causing more harm out of an even deeper sense of insecurity > rinse and repeat.
This is what the Kabbalists call your soul's "tikun" - the various imperfections your soul was born with, and the process of healing them one by one by virtue of being exposed to them in a mirror-like manner. Life forces you to encounter that which you are insecure of as a way of dealing with it. You know, kind of like what psychedelic work tends to do
That's just one perspective though, I'm sure there are many more interpretations and belief systems that explore the concept of karma. I personally believe in doing and being good. Not out of some righteous fear that otherwise I will be punished, but because it feels good to be a genuinely kind, good human being. I suppose there's always a portion of selfishness in being kind to others simply because it feels good, but still, if at the end of the day my actions and modus operandi make the world a marginally better place, then I know I'm living properly.