There are differences between the imams and the Holy Prophet that are mentioned in the ahadith, but in order to keep this article short, we will only mention a summary of these differences. For more information, one can refer to different exegetical and theological sources in this regard.
First: It is understood from the ahadith that an imam does not have the right of Tashri’ (legislation of legal rulings). Also, the rulings of the Shari’ah meaning what is haram and what is halal are not revealed to him. All of the rulings of Islam have been revealed to the prophet and even though the prophet only informed the people of those rulings that they needed, he also taught all of them to Imam Ali (as). Similarly, the imam passed them down to the next imam through speech and writing. These rulings were passed down from one Imam to the next in a similar form until now that they are finally possessed by the twelfth Imam.
Second: A prophet is connected to the hidden world at such a high level that allows him to see the angel that delivers the revelations and see the malakūt of this world, but the connection of an imam is not at such a high level. An imam does not see the angel, but rather the meanings are inspired to his heart, or he can hear the voice of the angel and through this way understands the truths and meanings.
Third: Prophets can directly be in touch with the unseen world and receive knowledge and are in no need of the help of any individual in doing so, while the Imams were in need of the prophet’s help and guidance to do so. 
Related Question: Imams and Takwini Wilayah, question 9343 (website: 9326).
 See: Bar’resiye Masā’ele Kolliye Emamat, Amini, Ibrahim; pp. 237-241.