There are five different endings you can get when you beat the game, three from the main game and two more added through the Godmaster free DLC pack.
The Hollow Knight – Defeat the Hollow Knight without the Void Heart.
Sealed Siblings – Defeat the Hollow Knight after getting the Void Heart.
Dream No More – Defeat The Radiance after getting the Void Heart, by using the Awoken Dream Nail on the Hollow Knight.
Embrace the Void – Beat all of the Pantheons and then defeat Absolute Radiance at the end of the Pantheon of Hallownest.
Delicate Flower – Beat all of the Pantheons and then defeat Absolute Radiance, but also give the Delicate Flower to the Godseeker first.
There is also an optional post-credits scene in addition to the endings. To unlock the post-credits scene, you must complete Mister Mushroom’s quest. This will display a short scene following any ending.
There are seven abilities you can unlock to allow you access to new locations and sometimes find secrets.
Mothwing Cloak – You get this by defeating the Hornet, and it lets you dash.
Mantis Claw – Once you have the Mothwing Cloak, you can use it to access an area in Mantis Village and find this ability, which lets you grab whiles and jump from them.
Crystal Heart – This is located after a platforming challenge on Crystal Peak and lets you propel yourself forward.
Isma’s Tear – Once you have the Crystal Heart, you can use it to access an area in the Royal Waterways and find this ability, which protects you from the acid in Hallownest.
Monarch Wings – You get this by defeating the Broken Vessel, and it lets you double-jump.
Shade Cloak – You get this by standing (and waiting) in the shadow fountain in the Abyss, and it lets you dash through enemies and attacks without taking damage.
Dreamgate – Once you have 900 Essence, you can talk to the Seer at the Resting Grounds to get this ability, which lets you place a Dream Gate at any location (except for a few specific locations that disallow use of this ability).