Herb vs. ‘erb:

The letter H comes from the Roman alphabet. The (h) sound was eventually lost in Latin, and in the Romance languages, which are descended from Latin, h is not pronounced although it is retained in the spelling of some words. In both Old English and Middle English, h represented (h) at the beginning of words and before vowels, although in Middle English h “weakened” somewhat and was often silent before vowels in unstressed syllables. In Modern English, h is usually pronounced in native English words such as happy and hot and, because of the influence of writing, in most words borrowed from French, such as haste and hostel. In a few other words borrowed from French the h has remained silent, as in honor, honest, hour, and heir. And in another small group of French loan words, including herb, humble, human, and humor, the h may or may not be pronounced depending on the dialect of English.

By request. More at World Wide Words.