Ogrines come in two forms: linked and unlinked. The only difference being that linked Ogrines can't be sold on the Ogrine Market for Kamas.
As with Kamas they have no weight, though an account can only have 180,000 Ogrines at a time.
There are two general ways to obtain Ogrines. The first is to buy them from Ankama. Ogrines obtained like this are not linked, and so can be sold on the Ogrine Market, as well as used for all other Ogrine services.
Alternatively they can be bought in game for Kamas on the Ogrine Market (the shortcut to access it is r). Ogrines bought like this are linked, meaning that they cannot be sold again, but can still be used for all other Ogrine services.
They are also sometimes given out as prizes for contests from Ankama, or alongside other Ankama products (such as the Dofus 2.0 Collectors Box).
Once Wakfu was released on Steam, Ogrines purchased with real currency were changed to no longer have an expiration date and will remain on the account indefinitely once purchased.
However, Ogrines bought with Kamas will expire at least three months after purchase depending on the quantity bought. The exact date of expiration can be confirmed under the Account Management section on the Dofus site.
When you purchased Ogrines from Ankama, you used to receive an extra bonus, Gift Points. You received 1 Gift Point per 100 Ogrines bought. These Gift Points could be used at the gift section to obtain additional rewards, such as special Pets and Livitinems.
The Gift Point feature was removed on February 22, 2012