Announced on November 9, Overwatch will be free to play across all platforms November 18-21. This is the second free weekend of play since the game released. In September there was a free weekend of play that was limited to X-Box 1 and Playstation 4 users.
The full roster of 23 heroes and all 13 maps and modes will be available that weekend. No features are locked behind a pay wall; users will be free to still collect loot, experience, and level up. All that disappears after the weekend, however, unless you decide to purchase Overwatch, in which case all you need to do is use the same credentials you logged in with to retain your goodies.
You can learn more about the free weekend in its blog post, including a deployment schedule, installation instructions, and an FAQ.