Risāla means "message" in Arabic. It is also an Islamic term that has a broader meaning.
The Message (Arabic ar-Risāla) is sometimes a way to refer to Islam. In the Islamic context, ar-Risāla means scriptures revealed from God through a Messenger (Arabic ar-Rasūl) to the people. These messengers bring laws to their communities so as to guide them towards the straight path of God. They can be social laws, state laws or any other kind of laws.
This is also the title of a popular work of Fiqh for the Maliki school of thought written by Ibn Abi Zayd of al-Qayrawan. This work was the source text for many later works of Islamic Law and for one of the first Islamic texts the Bilali Document written in the United States written by Bilali Muhammad, a slave in Georgia, originally from Timbo, Guinea.
Prophets (Arabic an-Anbīya) are the link of humanity to God through the Angels. They have specific functions according to the need of the time.
The relationship to Angels
Angels are the message carriers between God and His people. They are strongly connected to prophets and messengers, as well as miracles. They can appear to non-prophets such as Maryam, Sara and Luqmān.
On Laylat-ul-Qadr, Muslims believe that angels might appear to some pious people.
Al-Risala by Imam Ash-Shafi'i Ar-Risala by Imam Ibn Abi Zayd Al Qayrawani