Customs and manners vary greatly from country to country.

Literal

Customs-and-manners [topic-は] country [depending on-によって] big difference [subject-が] exist.

風俗習慣 is a compound: 風俗 (customs/manners) + 習慣 (habits). 大きな違いがある means 'there are big differences.' 国によって uses ~によって for 'depending on the country.'