Infusions are a type of upgrade component that go into infusion slots, which can be found on all ascended and legendary equipment (except amulets).

The Types:

These are Offensive Stat Infusions and also give +1% damage against Guards, Lords and Supervisors.

These are Defensive Stat Infusions and also give -1% damage from Guards, Lords and Supervisors.

While it’s not required, you might want to invest some time into Fractals to gain mastery in Fractal vendors to be able to buy an Infusion Extraction Tool so you can keep up with meta changes.
This also opens up another revenue for acquiring stat infusions.
For More information, check the Wiki Articles starting with >Infusions<

Maximum values

With the +5 WvW infusions, you can have +90 of your chosen stat, assuming you have 18 slots available on Ascended/Legendary armor and trinkets and -1% Damage taken from Guards, Lords, and Supervisors.

You will need to Infuse and Attune rings to get 3 slots on them (Fractal Mastery, sorry!), but you can buy an infused ring Directly from Skirmish Supervisor saving you half the effort.

Attuned or Infused can be upgraded to Attuned ring (infused) easily, have a look at the Wiki for instructions: >Attune/infuse<

But what about my Amulet?

Well something different happens here, its an Enrichment and not an Infusion what goes on that.

You get a choice of Experience/WvW-XP, Karma, Magic Find, Reward Track and Gold find.

These are on sale at any Laurel Merchant, and are ex-tractable using the Infusion Removal Tool from the Fractal Lobby