Comparative are grammatical forms used to express comparisons. Most commonly, this is adjective+er+than. E.g., My brother is taller than my mother. For some adjectives, more and less are used instead of -er. E.g, This city is more beautiful than my home town. (See Units 4 and 7).
"(Not) as...as " is another common comparative form: My mother isn't as tall as my brother.