A consonant is a speech sound that is produced by obstructing the airflow in some way, typically using the tongue, lips, or throat. In the English alphabet, consonants include letters such as B, C, D, F, G, and so on, as opposed to vowels, which are A, E, I, O, and U. Consonants can be voiced, meaning they involve vocal cord vibration, or voiceless, where there is no such vibration. They play a crucial role in forming syllables and words in various languages.