If you write a novel, you own the copyright. It won't be in the public domain for at least 10's of years after your death (typically 50-70 years), the details vary between countries.
To get the 70% deal you have to set a list price between $2.99 and $9.99, agree to let Amazon sell in all markets (non-exclusive world-wide rights), and agree to take 70% of the
sale price which is set by Amazon (between your list price and $2.99) and could change. You also agree not to sell for less anywhere else and to set the price at 80% or less of any physical book version.
With the 35% deal, you get 35% of the
list price, which has to be at least $0.99, and you can add geographic restrictions if you want.
Amazon's
Pricing Page and
List Price Requirements provide more details.
The $2.99 requirement for 70% has been a problem for some authors who also sell on other sites like Smashwords, because they had no control of the selling price and Amazon would switch to 35% if this went below $2.99. This is why Smashwords is going to Agency pricing, where they (i.e. their authors) set the selling price.