The regular form of comparative adjectives (better, faster, smarter, worse, etc.) is made by replacing the ending with -ее. Some adjectives (including ones where the normal neuter ending is -ее instead of -ое and ones where the stem ends on a д к х т or г), however, have irregular forms. Here is a short list of them. I doubt this is all of them, but these are very common ones.

ранний - early
раньше - earlier (sooner)

поздний - late
позже - later

большой - large
больше - larger

маленький - small
меньше - smaller

краткий - short
кратче - shorter

долгий - long
дольше - longer

молодой - young
моложе - younger

старый - old
старше - older

далёкий - far
дальше - farther

близкий - near
ближе - nearer

хороший - good
лучше - better

плохой - bad
хуже - worse

дорогой - dear (precious, expensive)
дороже - dearer (more precious, more expensive)

дешёвый - worthless (cheap, modest)
дешевле - more worthless (cheaper, more modest)

I don't know why the ё disappears in the last one.