NXT ist the original blockchain, created in late 2013 or early 2014. Today it could be called a legacy chain, but anybody can use it for the purposes they want.

ARDR is the main coin of the successor blockchain (originally called NXT 2.0) started in early 2018. 1 ARDR was granted to all NXT holders at a specific snapshot.

IGNIS is a token on the ARDR chain which uses the child chain mechanism and is meant to be used for payments, minor tokens, and so on, as its transaction fees are meant to be lower than ARDR's and its transactions can be pruned.