Ooh me me me!!
Tubing is just that. A length of tubing that the main line runs through. Can be a pain to thread in long lengths. At the end of the tubing is tied the lead arrangement and hooklink.
A leader is a length of material, various available, which the main line is tied directly to. The lead arrangement is usually part of the leader, or fixed to it, with the hooklink attached to the end.
The issue many have with leaders is that in the event of a snap off, the line will usually break at the knot. If using tubing the fish is only left with the hooklink when used correctly. With a leader the fish is left with the leader and hooklink, and depending on the lead setup, the lead too. The tubing option is undoubtably safer in most setups and situations. Arguably it makes no difference/can be safer with a leader when fishing a rotary rig, but I won't bore you with that!