They can indicate that a word is used in a certain region, for a specific subject, or that it has a special usage slang, informal, nonstandard, archaic, obsolete, vulgar and so on. Use the dictionary to hunt around for synonyms of words. Although it's not as handy as a thesaurus, you will find plenty of related words by doing multiple lookups using the words in definitions. Not all words are in any one dictionary. Run-ons and derivatives are forms of the base word such as electricity from electric.
Inflections are grammatical changes in number, person, mood or tense and plurals.
You may be surprised to learn that there are variant spellings and pronunciations. The first listing is not necessarily correct but, rather, preferred. Remember that the dictionary is not an unquestionable authority. It is written by trained professionals reporting on the real use of words and phrases by the general public. The use of the name Webster is public domain.