Internal audit nothing but the checking the product that you produced. External audit is checking your product by your customer. Here it means you may not find mistakes in your processes but a third man who comes and check the system he may see some deviations in the system and give suggestions for the improvements of the system.
Internal Audit is a examination of day to day activities like verification of sales/purchases/payments made by a person employed by company with in & it is carried mainly to put good control systems in place & mistakes to be soughted after across the table in no time.
External audit is carried out by a CA to ensure True & Fair view of the company for the outsider like share holder's Creditors Govt authorities, Banking authorities.