They were compilation albums of "greatest hits." I forget which is which, but one covered the years 1962-66 (the 'blue' one, I think), and the other covered '67-70.
Wow this is a long answer... The Red Album is a compilation of Beatle songs from 1962-1966 It has the EMI pose from Please Please Me (1963) The Blue Album is a compilation of Beatle songs from...
The formal titles of the red and blue albums are The Beatles 1962-1966 and The Beatles 1967-1970. They were issued as "official" versions of a bootleg mail-order compilation. Both albums were...