Explorer is MTG Arena’s closest equivalent to the Pioneer format that you can play on tabletop or Magic Online. As time marches forward, the goal is for the two formats to come ever closer to converging. The latest effort towards this is Explorer Anthology 1.
You can consider Explorer to be “Pioneer lite.” The two formats are pretty close, but Explorer is missing a key handful of cards, which does change the nature of some of the top decks in the competitive metagame.
Prior to the release of Explorer Anthology 1, Martin Juza wrote a Deep Dive that offered an excellent background for someone new to the format.
Additionally, Frank Karsten wrote a piece that highlighted the key differences between Pioneer and Explorer.
Of the cards Frank highlighted, a modest handful were printed in the Anthology, but many are still missing. Let’s briefly mention what still isn’t available on Arena.