Gods are separated into different tiers. Some require 10 shards to summon, some 30, and others 80 shards. The upside to collecting 80 shards to summon a god is that they are already at 3 stars. Similarly, gods that require 30 shards to summon come at 2 stars.
Receving a hero you already have converts the hero to shards. 1★ heroes are converted to 7 shards, 2★ into 18 shards, and 3★ into 30 shards.
To ascend a hero to two stars, it requires 20 shards. To three stars requires an additional 50 shards. The real only differnece between a one star hero and a three star hero is how soon you can start using and leveling it. Three star heroes are much rarer than one star heroes. Eventually, all heroes can reach 5 stars.
Heroes are separated into different groups. These groups are what Stats they are based off of: INT, STR, and AGI. They also have 3 Positions they can be in: Front, Mid, and Back. Lastly, Heroes are divided into 5 Roles in a team: Tank, Nuker, Disabler, Support, and Healer (however some heroes take on 2 or 3 different roles at once, and some roles better than others).