Short Tail Keywords are usually between 1-2 words long. These keywords have the Highest search volumes with Low relevancy, and are the most competitive.
Mid-Tail Keywords are typically between 2-3 words long. These keywords have High search volumes with the Good relevancy, and are quite competitive.
Long-Tail Keywords are typically between 3-5 words and longer. These keywords have Low search volumes with the Highest relevancy, and are the least competitive.