Taller is a comparative which means the word that is used to compare with other people.
Jason is tall.Randall is short.
Jason is taller than Randall(is).
Tall and taller are both adjectives.
The difference is that tall is positive and taller is comparative.
Positive comparative Superlative
Tall Taller Tallest
Clever Cleverer Cleverest
Intelligent more intelligent most intelligent
beautiful more beautiful most beautiful
'' less beautiful least beautiful
Words that are more than two syllables are accompanied by more/most or less/least.