Hi,
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!
(edited)