The main difference between the Intro Membership and Permanent Membership is the commitment of preservation. Permanent Members pay into the Forever Guarantee Fund which ensures that their storage is protected and accessible in their lifetime plus 100 years. Amounts of storage range from 10 GB to 1 TB.
Intro Members, on the other hand, are granted 1 GB of storage and are NOT guaranteed permanency of their account's content. However, the actual functionalities of the site--sharing photos, curating albums, etc--does not differ between the two types of memberships.