Let's say you have three parties:
Marmite, Vegemite and Peanut Butter
Vegemite lovers hate Marmite and vice versa. Peanut Butter isn't loved by as many, but isn't hated either.
You have 249 voters. Everyone ranks their preferences in order.
100 people vote marmite 1, peanut butter 2, vegemite 3
99 people vote vegemite 1, peanut butter 2, marmite 3
26 people vote peanut butter 1, vegemite 2, marmite 3
24 people vote peanut butter 1, marmite 2, vegemite 3
FPTP: Marmite wins, because only one vote per person counts, and more people voted marmite than anyone else (100->99->50).
AV: Vegemite wins, because after peanut butter is knocked out of the running, and the second-place votes are transferred, it has 125 votes to marmite's 124.
Condorcet: We compare each option to each other one, and see which one wins. Peanut Butter beats Marmite (149->100), Peanut Butter beats Vegemite (150->99), Vegemite beats Marmite (125->124). As Peanut Butter wins all of its battles, it's the victor, Vegemite is in second place.
Which of these three systems would you rather have, based on the above?