It is confusing and I'm not exactly sure how they work myself. Best advice I can give you is CP is a general overview of a Pokemon's level while the IV is the actual stats of a pokemon

For example you can have 2 mewtwos one perfect IV and one with 0 IV. Both can be powered up to around 2400-2500 CP for the ultra league. The perfect IV will do more damage have more health and have higher defence even though both have same/similar CP.

So to give a simple answer CP= level, IV = HP, attack, Defence

Every pokemon type has a different max CP value, Mewtwo is somewhere around 4200 and a shuckle maxes around 450. So each pokemon has a league it preforms best in essentialy.

If you look at powering up a pokemon you can see a point where you need XL candy to level up any higher. This would be considered lvl 40 trainer level and as your trainer level goes up you can take your Pokemon's CP higher.

If you have anymore questions I can try and answer them better if this is confusing.