Ok so Nephalem rifts are random areas that have random enemies. They have a better loot drop chance and at higher levels can drop Greater Rift Stones once you beat a boss that appears when you finish filling the progress bar that appears when you first enter. The greater rift stones are used to enter Greater Rifts. These are used to access rifts that have to be beaten within 15 min. They drop NO loot during the rift but once you beat the boss you get special gems that give great bonuses when socketed into amulets or rings and can level those gems up this way as well. the experience in these GRs are also much higher and have better drops at the end. Greater rifts can also go above the Torment X difficulty.
Paragon Levels are infinite, but you will max out the capable bonuses at lvl800. After that it is just main stat and vit.
You should be working to do rifts and getting legendaries to suit the build you want, Greater Rifts to get the gems you want and level them up, Bounties can be done to get crafting materials for kanai's cube to extract powers once you get what you need.