Children are classified as having a speech delay if they are progressing through the proper development sequence but at a slower rate than other children. (Kathryn Thorson Gruhn, a speech language pathologist)
There are a number of different disorders that fall under the umbrella of “speech delay.” First are the speech disorders, which can be further divided into articulation disorders, fluency disorders and voice disorders. Articulation disorders refer to children who have difficulty making different sounds, while fluency disorders (including stuttering/stammering) are where the flow of speech is disrupted by involuntary repetitions and prolongations of sounds, syllables, words or phrases as well as involuntary silent pauses or blocks in which the person who stutters is unable to produce sounds. If your child has trouble making verbal sounds, it might be a voice disorder.
The other type of speech delay is a language disorder. With a language disorder, the problem has more to do with understanding the meaning and use of different words and phrases rather than the ability to say those words. When a child has trouble understanding others, it is called a receptive language disorder. When he or she has trouble sharing thoughts, ideas and feelings, it is called an expressive language disorder. (Sharon Willig, (ASHA)).
Children devlop at different speeds, so it is sometimes difficult to tell if a problem exists or not. If you are concerned, it is always worth just having it checked.