Tier S: Very Strong
This category wouldn't exist if Blizzard's balancing was better, but this small club of Heroes generally deserves a first ban or first pick with relatively few exceptions (like if you see an enemy One-Trick-Pony who's godlike at Cho'Gall or Zeratul and are daring enough to first pick them for instance).
Tier 1: Strong
I think of these Heroes as having a clear path to be played, a decisive play style that has a clear win condition. In this sense, I see them as Heroes you can build a team around more easily than the lower tiers. They are generally quite strong or versatile.
Tier 2: Viable
The most important condition about these Heroes, is that they should not be first picked, as they have pretty solid counters. Middle or last pick is totally fine. They are all viable, but miss the cut for Tier S or Tier 1 because they are not stupidly broken, or universally versatile - i.e. practically uncounterable.
Tier 3: Usually Not Optimal
These Heroes are not grief, but they are either way more situational, very difficult to play correctly, or just a little underpowered in the meta because better alternatives exist. Every now and then, you'll find a spot to play them in though, and that can be fun, effective and satisfying!
Tier 4: Weak
"But I won a game with a Weak-classed hero!" - Yep, all Heroes are technically winnable! There are countless factors such as personal skill and preference, enemy mistakes, enemy mis-drafts, allied misplays etc.! These Heroes all have much better alternatives in the Nexus, but if you love them and want to play them, who's stopping you, aside from a few reports?