Adjectives are words used to describe tangible or intangible attributes of nouns. Of course, if you just give this definition to students learning English as a second language, they can end up more confused than before. Instead, use practical examples to help them understand what is an adjective and how it's used, as well as allow them to practice their skills on a variety of exercises and games.
Begin the lesson by showing your students a number of pictures containing obvious contrasts, such as one short person and one tall person, a happy face and a sad face, a round object and a square object. Unless students are aware of the words "tall," "short" and so forth, explain the words first and then ask students to tell you what is the common element of all pictures. When they answer that all images contain contrasts between people or objects, it's time to move on to the lesson.
Explain that, without adjectives, we would not be able to explain the difference between the people or objects depicted. "Tall," "short," "fat," "thin," "shallow," "deep," "happy" and "sad" are all adjectives, words that help us distinguish nouns based on qualities we can identify. In case your ESL classroom is composed of young kids, use simpler vocabulary when explaining the concept of adjectives.
Group games are a way for students to practice their skills in identifying and using adjectives under the guidance of the teacher. Write a few sentences on the board, such as "The cat is fat" and "The red bus is early," and ask students to work in groups to spot the adjectives.
For another vocabulary game, provide photos of people with obvious facial expressions -- happy, sad, confused or shocked, for example. Write sentences on the board to go with the pictures. For example, write, "John is a sad man. He lost his wallet." Ask students to match the sentences with the pictures.
Prepare worksheets to distribute to students so that they can practice their skills individually. This way, you can also identify the weaknesses of each student. You can find a wide variety of worksheets at ESLFlow.com and Super Teacher Worksheets (see Resources). These worksheets include exercises requiring students to find the adjective, identify the noun it describes, or match pictures with the corresponding adjectives. In addition, you can assign a few worksheets as homework.