Both expresse the inner (part) of something but they differ in usage. Internal usually means located or effective within the body, the intrinsic, or the mental or spiritual nature. Insider refers to the member of a (specified) group or an organization, a member who has a special knowledge or exclusive information or has some special advantage or influence. Internal also refers to domestic politics of a country, the internal affairs. Insider, similarly to the description above, refers to a person having a position in a business or a stock brokerage which allows them to possess confidential information.
