Lots of great explanations below, but I fear they might be difficult to truly conceptualize.
Let's start with a pie!
If there's a pie in front of us, that is the market. We'll call it 100 units of pie
We cut the pie in half - one piece (50 pie units) for you, and one piece (50 pie units) for me
Scenario 1: We do nothing, we both have 50% market share.
Scenario 2: I eat half my pie. My pie slice is now 25 units, yours is still 50 units. As a result, you have 67% of the market share and I have 33%. My "revenues" have fallen AND my market share has fallen. Your revenues haven't changed BUT you've gained market share
Scenario 3: Not only do I eat half my pie, but you also eat 20% of yours. Still, I ate more, so even though your "revenues" are down, you've still gained market share.
Scenario 4: We both get given more pie (yay). BUT, I get a whole other 50 units, and you only get 5 more. Your "revenues" are up, but you've still lost market share!
I've created the below image for you to visualize this better (click on it to blow it up)...hope this helps!