Language is a system of communication in which the meaning of the sounds used by a particular group are understood by others who share the language. Sharing a language often means mutual intelligibility, or in other words, you talk to me, and I can understand most of the words you're saying, and vice versa.
A dialect is a variation in a language and may include different ways of pronouncing a word, different spellings, or different choices of words. Various regions will have different dialects. Accents are often part of different dialects. An accent is the way that we sound when we pronounce words. It's important to remember that we're all speaking a variation of a language and that we each have our own accent and the capacity to change ways that we speak if we are exposed to other dialects.
Experts in linguistics look at how languages evolve in this way and how different regions have developed their own patterns of speech.