Actinobacteria, which is a subgroup of Gram positive bacteria, are the reason why it smells good after it rains. These filamentous bacteria produce spores during dry conditions and then the rain moves around these spores which has a distinctive, earthy scent. 

This bacteria is very abundant in soil. The long, threadlike, branched filaments in compost piles are formed by this bacteria. 

Earthy smell is also due to a compound called geosmin that is produced by fungi and bacteria, particularly Streptomyces coelicolor