Many popular annual bedding plants have potential to be perennial if they get a chance. Cyclamen commonly perform as a cool season annual, only until replacement with warm season annuals. However, after summer dormancy, their tubers can generate foliage and bloom again for several following winters. Replacement is just easier than maintenance.
Real annuals complete their entire life cycles, from germination to dispersal of seed and finally death, within a single year. Biennials generate vegetative growth through their first year, and then disperse seed and die during their second year. Perennials perform for at least two years, whether or not they get any opportunities to do so within home gardens.
Home garden culture complicates this classification though. Those who enjoy gardening expect perennial plants to reliably perform for many years or indefinitely. Many perennial plants do so with only minimal intervention. Some are self-sustaining, and might seem to naturalize. That may be why so many smaller or less vigorous types classify as annuals.
Another qualification of perennial plants is that they lack woody stems and roots. In other words, they are herbaceous. Palms and species of Yucca that develop trunks classify as perennial plants also, but for simplicity, the larger types are herbaceous trees. Generally, perennials are terrestrial. A few are epiphytic (live in trees) or lithophytic (live on stones).
Although several perennial plants can survive indefinitely here without intervention, most perform better with some degree of attention. Most ferns are neater if groomed to remove deteriorated fronds. Many grasses benefit from severe shearing during winter. African iris blooms better in response to regular deadheading to remove developing seed capsules.
Perennial plants are very diverse. It is impossible to generalize about their maintenance. One commonality among many is that they multiply. Lily of the Nile can produce so many individual shoots that it can eventually become too congested to bloom. Division of such shoots every several years or so promotes bloom, as well as propagates more plants for elsewhere in the landscape.