synchronic linguistics studies a language at one point in time and diachronic studies a language over a period of time. You might say that synchronic linguistics is a photograph and diachronic l. is a movie. Usually synchronic l. focuses on a language in the present time whereas diachronic l. concerns itself with the history of the language, how it developed what changes occurred.

Synchronic linguistics is the study of a language in a given point in time. Diachronic linguistics is the same as historical linguistics, it´s the study of a language over a period of time.