A 12-bar blues progression is composed of three (typically) four-bar phrases. The first phrase is entirely tonic harmony (I). The second phrase contains two bars of subdominant (IV) and two bars of tonic (I). The final phrase begins with one bar of dominant (V) followed by one bar of subdominant (IV) and two bars of tonic (I). The third phrase may or may not end with a turnaround.
I / / / IV / I / V IV I /
Because the first phrase starts with I, the second with IV, and the last with V, we can call these phrases the tonic, subdominant, and dominant phrases.
A 16-bar blues progression is composed of four (typically) four-bar phrases, usually two iterations of tonic, followed by subdominant and dominant. The final phrase may or may not end with a turnaround.
I / / / I / / / IV / I / V IV I /
Of the two, 12-bar blues is more common. And though both can be found in modules of all types of functions, blues progressions are most typically found in strophes (both in strophic and in AABA song forms).
Frequently, songwriters will make alterations to the standard harmonic pattern or extend/compress phrases by a bar or two. However, if you hear most of the features above, consider it an altered blues progression and use the standard 12- or 16-bar pattern as a reference for listening to what specific details have been altered.
A straight 12-bar blues progression can be found in “Hound Dog” by Elvis Presley.
“Don’t Be Cruel” by Elvis Presley presents a 12-bar blues pattern with an alteration of the final phrase (II–V–I rather than V–IV–I) in the strophes (the song is in AABA form).
“Surfin’ USA” by the Beach Boys presents a 16-bar blues strophe with the two first phrases each beginning on two bars of V before two bars of I (V / I / instead of I / / / ).