Adverbs in French

Informally, adverbs are generally "optional" words that indicate manner, and related notions such as time, location, extent.

A bit more formally:

Adverbs are words that can generally fill the gap in the phrase Very _____ and modify something other than a noun (including the whole sentence).

Typically, they can appear at the start or end of the sentence, or (in English) between the subject and verb. For example:

(Very) soon/recently/carefully, she came back to the house.
She (very) soon/recently/carefully came back to the house.
She came back to the house (very) soon/recently/carefully.

Suggest a change / proposez une modification

There are words such as well that we also class as adverbs even though they can't appear in all three positions, or words such as presently, which tend not to be used with very, but which can otherwise occur in these three positions in the sentence.

Adverbs in English tend to be either:

based on an adjective plus the suffix -ly: carefully, hopefully, desperately, firmly, newly, recently;
be a (usually short) word in its own right— typically, these relate to time or location: soon, here, there, then, well.

Traditionally, grammars and dictionaries have also used "adverb" as a dustbin category: items that aren't easily analysed as one of the other traditional categories have been lumped in with adverbs, including notably WH-words (when, where, who etc). We're not dealing with such words in this section.

In French, the situation is somewhat similar:

many French adjectives can be turned into adverbs by adding the suffix -ment (roughly equivalent of English -ly);
there exist a few, generally short, words that are adverbs in their own right (the equivalent of English soon, here etc).

formation of French adverbs: a look at using the suffix -ment to form adverbs from an adjective, including a few complications and irregularities.

French grammar index
French-English dictionary
English-French dictionary