People understand Karma as some form of the Golden Rule ("treat others as you'd like to be treated") which is more practical - with deeds and rewards. That is, the Golden Rule is a way of positively interacting with the world, and Karma is thought of as some metaphysical "law" that will provide you with positive returns if you give your positive energy.
In the big picture, the truth about Karma is difficult to pin down, given the vast variety of ideas on it between the cultures of its Eastern roots. The earliest known texts outlining Karma are the ancient Hindu Upanishads:
Now as a man is like this or like that,
according as he acts and according as he behaves, so will he be;
a man of good acts will become good, a man of bad acts, bad;
he becomes pure by pure deeds, bad by bad deeds;
And here they say that a person consists of desires,
and as is his desire, so is his will;
and as is his will, so is his deed;
and whatever deed he does, that he will reap.
- Brihadaranyaka Upanishad
Many people in our world feel that the world has cheated them - that they've been beaten down senselessly to no end, all while feeling that they've done right by the people around them. "If this is so, how could Karma possibly be real?" they will ask you, and themselves, over and over again. I pose that the answer to their question is that Karma is real, but not necessarily definite. That is, good Karma (good action, deeds) doesn't necessitate good returns, it only makes it infinitely more likely that you will have positive repercussions. In my mind, this puts a true sense of Karma much closer to our concept of the Golden Rule. I'll explain this in a simple metaphor:
Let's say you're walking down a hallway at home, work, school, anywhere you might see people. With every set of eyes that you might meet with your own, you have a choice - to smile, to wave, to greet them, to say something rude or passive aggressive, or to divert your eyes, to name a few. Each time you make one of these choices, you're sending a message to them whether positive, negative, or neutral.
Lets say you choose to smile and wave when given the opportunity. Here's the kicker: now they have a choice - to respond or ignore you. If they choose to smile and/or wave, then they have observed your action and responded in correlation with what we know as Karma. If they don't, then a positive energy you've given to the world goes uncompensated and you may feel like it didn't make a difference.
Here's the thing: it probably did. That person on the other side of the hall has their own struggles and worries, and they might not have responded out of internal conflict. They may be dissociative or tired. But no matter the reason they appeared to ignore you, chances are that your smile and wave will sit in their heart for awhile. They might think of you later, and appreciate your kind hello for what it was - a simple, positive gesture. They might pass on this energy the following week when they meet someones eyes in the hall, and smile and wave when they might not have before.
The other option? You shed negativity wherever you go, and it becomes a part of you and the people you interact with. You still have a chance of positivity entering your life - the universe works in mysterious ways, as they say - but it is much more likely that your negative output will come back to you at the end of the day (month, year, century, whatever) and cause a form of 'suffering', so to speak.
In conclusion, my experience is that Karma is real - it might not come back to you with recognizable returns soon, or later, or at all for that matter. Your Karmic energy might go through a person to someone else, never allowing you to see your efforts (or wrongdoings) come to fruition. Not getting rewards from Karma directly does not mean you aren't contributing to the Karmic energy pool every day.
Of course, providing positive energy in times of extreme hardship or depression is always easier said than done. Regardless, when the weight of the world sits heavy on your shoulders, and you feel like Karma has failed you, falling into the trap of constant negativity and hopelessness is likely to only reinforce this downward spiral. If possible, my suggestion is to do your best to give the hope, love, and support that you want from others no matter how futile it feels - Karma says that it is much more likely that you will find yourself back on track navigating towards happiness, grace, and stability.
Thoughts?