Wait, why is there another new Spiderman reboot / recast / reimagining? What was wrong with the last kid?
Disney/Marvel wanted Spiderman in their films, but Sony owns the rights to him. They worked out an agreement where they can both use Spiderman and decided it was in everyone's best interests to use the same actor, hence another reboot. Previous Spidey was an older version and Stark bringing in a teen Spidey was a better fit for the story Marvel wanted to tell. Tom Holland (age 20) plays Spidey in Captain America Civil War (Marvel Studios) and in Spiderman Homecoming (Sony Pictures). Andrew Garfield (previous Spidey) is 33.
It's a simple, but brilliant deal in my opinion. Disney didn't have to pay a dime for him - each studio just shares the rights and keeps 100% of the profits from the films they release. It's a win-win for everyone - Marvel gets to bring him into their Avengers universe and Sony gets to feed off Marvel's other successful franchises to help make their Spidey movie a bigger success.
The only odd part is RDJ as Iron Man in a Sony film - I knew he was in it, but figured it was a cameo. Sure looks like a much bigger role. I'm a little surprised that Sony gets to use IM, but apparently Marvel negotiated creative control of the film, so it's a lot more like a Marvel Studios movie released under the Sony name. I wish Marvel and Fox had been able to work out something similar for the Quicksilver character instead of each going their own way.