A phrase is a group of words that are together as a single unit. It can appear as a part of a clause or a sentence.
A phrase does not contain a subject and verb and, so it cannot make a complete sentence. A phrase is not like a clause. A clause does contain a subject and a verb, and it can make a complete sentence.
1. As soon as you got in, he went out.
(As soon as is a conjunctional phrase.)

2. Popcorn popping, the dog was ready for the film.
Popcorn popping” modifies the clause “the dog was ready for the film.”

3. The entire rugby team, their uniforms muddy and stained, shouted in joy.
A phrase will have more than one word. Hence, the words like homesick, thoughtless, and such are not phrases. They are simply words. Although the word "homesick" is a combination of two words-- home and sick--it is still not a phrase.
Examples of phrases from the lesson "A Different Kind of School":
1. I asked her some questions on her teaching methods.
2. Miss Beam was full of authority.
3. No more than is needed to help them to learn how to do things. 
4. It pains me, though, to see that they are not all so healthy and active-looking.
5. Miss Beam was walking up and down.
6. I had heard a great deal about the school.
7. When I arrived there was no one in sight
8. She seems to be a hopeless cripple.
9. It is really something of a game.
10. They are on their honour not to peep.
11. But they are not as careful as I shall be when it is my turn.
12. Let’s go for a little walk.