Very expensive, blueprints contain all the necessary data for the manufacturing of various items. Blueprint copies have a limited number of runs and do not include the hard materials for manufacturing. Essential for any manufacturing process. They are not fitted modules, and are given to the station assembly plant for manufacturing. Unless you make all the runs at once, you will get back your blueprint minus the number of runs you set it for.
Blueprint originals have an infinite number of runs, but a maximum limit on runs that can be requested at in the same job. They can be traded on the market, while copies can only be traded through the use of contracts. Originals can be researched to improve the material effeciency and the time efficiency, making research a good long term investment. It is also possible to make copies from a blueprint.
[] is a helpful guide to Blueprints