The 21 consonants are: b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x, y, z
In English, no words can be formed with consonants only. Commonly cited as exceptions are words like sky and fly; although listed within the consonants, the letter y is considered a semi-consonant by some, a semi-vowel by others. This is because it can stand for a consonant as in "yoke" but also for a vowel as in "myth".
In terms of sound production, a consonant is a sound which includes some kind of blocking. For example, if you make the sound /m/ as in /mike/ the lips are closed. Compare this with a vowel where the air flows freely through the mouth without interruption.
Vowels and Consonants - a full look at consonants and vowels in English