Tesla's Gigafactory in Nevada is on track to be the largest manufacturing building in the world by footprint. It will help produce batteries for Tesla's automobile and other green energy related product lines. As if having the largest factory in the world wasn't enough, Tesla now has plans for several more.
A few months ago in an earnings call CEO Elon Musk revealed plans to announce 2 or 3 more Gigafactories this year. However, in a TED talk a few days ago, he bumped this up again when he said he "will announce locations for between two and four Gigafactories later this year - probably four."
Most of Tesla's business is currently in the United States so it makes sense that Gigafactory number 1 is in Nevada with number 2 coming in New York. Musk believes it would take 100 Gigafactories around the world to fully transition to sustainable energy, so Tesla alone will bring us 5% there if they continue this aggressive strategy.
Many European countries have been pressuring Tesla to build the next factory in their borders. Musk has agreed that Tesla "needs to address a global market" but wouldn't give any more details. It would also make sense to build a factory somewhere in Western Europe given their progressive policies on clean energy. China too is likely high on the list given Musk recently met with top officials there. The fourth factory is anyone's guess but I would bet it shows up somewhere in the central United States.
The original Gigafactory has a capacity of 100-150GWh of batteries at full output. We don't know if the future buildings will have the same capacity but with so much invested in the future of renewable energy, we can definitely expect great things to come from Tesla and Elon Musk later this year.