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
All of these Heroes perform quite well and are flexible. Some are difficult to target-counter, whereas others are strong numerically (stats). Most of these heroes excel in some way, like long range, global presence, saving people from blow-up, or setting blow-ups or team fights up really well. They are fine to be picked in the #1-3 draft slot.
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?