A Win Condition is a specific means or strategy by which a Team can achieve victory. Even though the main goal of Heroes of the Storm is always the same – to destroy your opponents' Core – there are different ways to achieve this goal. Your Win Condition starts out after the pick/ban phase during Draft.
Different heroes have different strengths and weaknesses. Having multiple heroes with similar strengths in one team can lead to a very different playstyle than you would have with another team composition.
If you draft a team composition around one or two hyper carries such as Illidan, Valla, Tracer, or Greymane, you are usually trying to win the game based off team fights. If you are instead picking up a lot of global pressure and wave clear, split pushing is your strength. In terms of power spikes, one of the best heroes to look at for a single explanation is Nazeebo. Nazeebo gains a lot of power the later the game goes. It’s fair to say that a Nazeebo before reaching level 20 is at a completely different tier in terms of strengths than after he finishes Vile Infection. It’s not uncommon that his power spike alone can change the outcome of team fights.
Understanding when your hero – or rather, your team composition – is at it’s best can be the difference between winning and losing a game. The simple difference between strength based on time and talents is early and late game, but you can break it down even further and compare the power your and your opponents team gain on every talent tier, starting with level one.