I'm a relative newbie at reloading but I'll take a stab at some of your questions. lands is where the bullet engages the beginning of rifling twist in your bore. Some shooters--like certain match calibers--will actually develop their loads so that the bullets are engaged into the lands upon chambering. But these are not your "everyday rifles" and generally most hand-loaders will seat their bullets with a bit of space or "jump" off the lands (though some do load right into the lands). The relationship is not entirely absolute--in other words while most bullet/powder combos can be improved upon (one of the main benefits of hand loading) by adjusting your bullet seating (usually by measuring distance to the bullet's ogive as opposed to the "conventional" overall cartridge length) so that they are closer to the lands--there are some bullets and chambers which "prefer" a generous jump.

Over-all cartridge length is simply a standard to provide a high degree of conformity of manufacturer's ammo to manufacturer's rifles. As hand-loaders we "push those boundaries" a bit in search of accuracy that out-performs the manufacturer's standards. Factors such as your magazine's length capacity and dimensions of your rifle's chamber will determine how far you can push those boundaries--along with the safety limitations of the powder loads within the case. General rules of thumb are develop your loads cautiously working your way up toward max loads--and be sure to examine your cases carefully and know the signs of incipient overpressure or other problems. A gun blowing up on you can ruin your day.