An adverb is the part of speech that describes, modifies or provides more information about a verb in a sentence. An adverb can also be used to modify or qualify adjectives, other adverbs, or whole word groups. Many adverbs in the English language end with the suffix -ly, since this is a quick and easy way to turn an adjective into an adverb. For instance, the adjective "sad" transforms into the adverb "sadly" by adding -ly to the end. The same is true where "perfect" becomes "perfectly."
The -ly suffix is also an excellent way to describe how something, or how often, something is done. While not a hard and fast rule, and there are certainly many exceptions, spotting the -ly ending in a word is oftentimes a good indicator that you're looking at an adverb. Many adverbs end in -ly, but not all of them do. Common adverbs that don't end in -ly include "very" and "never."