Mind you, when Blizzard say free Overwatch weekend, they actually mean four days starting on Thursday, 26 July and ending on Tuesday, 30 July 2018. So it's still technically a weekend of pure Overwatch carnage with 28 heroes across 18 maps.
Players who download and log into Overwatch between 26 and 30 July will have access to all the heroes, Hammond included, and all the available maps. Naturally, competitive play will not be accessible for obvious reasons but the rest is all there.
This means Quick Play, Custom Games and Arcade are playable and Blizzard wouldn't be Blizzard if they didn't advertise loot boxes as available. Of course, you'll be getting them in Quick Play anyway so if you happen to buy Overwatch by the end of the trial period, you get to keep your new shiny stuff.
Your stats will also be retained should you choose to buy Overwatch, while achievements are off limits until you do. Interestingly, the dev found it important to highlight no cross-play in Overwatch. They obviously get asked this a lot, since the game is available for PC, PlayStation 4 and Xbox One.
In case you're really new to Blizzard's co-op shooter, expect to be bombarded with Overwatch League content, since the competition's Grand Finals are taking place at the same time. On the other hand, it may be worth being bombarded about, because the finals are shaping up to be a can't-miss affair. No, not for Widow Maker mains, they'll probably miss it, since that's what they do best.
Note that the free "weekend" is only for PC players outside of South Korea. We guess Blizzard has no need for free Overwatch weekends there, what with the game being religiously followed in the country. Heck, its popularity even indirectly brought about legislative changes, resulting in trials to 13 people for coming up with software for cheating in Overwatch.
In related news, the Overwatch team have announced a support hero balance pass for the next PTR patch, nerfing Brigitte and Mercy while buffing Lucio, although it's highly unlikely to be applied on the live servers by the time the free weekend ends.
You can find the details of the free Overwatch weekend on Blizzard's official website here.