The main distinguishing feature of phrases is that they lack a subject-verb combination. They can certainly contain nouns and/or verb forms, but they are not joined as they are in a clause.
Phrases can be used as nouns, adjectives and adverbs.
A prepositional phrase contains a preposition. In the morning is a prepositional phrase.
A gerund phrase uses a verb as a noun (which is called a gerund). Walking alone is a gerund phrase.
An infinitive phrase uses the infinitive form of a verb. To go downtown is an infinitive phrase.
Depending upon how you use these different phrases, they can become nouns, adjectives or adverbs.
He exercises everyday in the morning–an adverb usage.
In the morning is the best time to exercise–noun usage (although leaving the in off this is better English–this is just an example!).
Coffee in the morning is a real waker-upper–adjective usage.