The verbs in both the following sentences are Intransitive:
1. The baby sleeps.
2. The baby seems happy.
But, the first sentence "The baby sleeps" makes complete sense, whereas the other sentence "The baby seems happy" does not make complete sense.
The Intransitive Verb seems require a word (e.g., happy) to make the sense complete. Such a verb is called a Verb of Incomplete Predication.
The word happy, which is required to make the sense complete, is called the Complement of the Verb or the Completion of the Predicate.
Verbs of Incomplete Predication usually express the idea of being, becoming, seeming, appearing. The Complement usually consists of a Noun (called a Predicative Noun) or an Adjective (called a Predicative Adjective). When the Complement describes the subject, as in the following sentences, it is called a Subjective Complement.
1. The earth is round.
2. Roses smell sweet.
Note: When the Subjective Complement is a noun, it is in the same case as the Subject, i.e., in the Nominative Case.
Certain Transitive verbs require, besides an Object, a Complement to complete their predication; as,
1. Her parents named her Priya.
2. The girls made Shreya captain.
Here, the Complement describes the Object, and is, therefore, called an Objective Complement.
Note: when the Objective Complement is a noun, it is in the Objective Case in agreement with the object.