Design briefs are very important to the design process because they help you to fully understand the project you are about to undertake. There purpose is to allow the designer to gather essential information from the client in a clear effective way and to help understand the aims, objectives and milestones of the project.
However, there are many different forms a design brief can take, for example it could be; an email, a phone conversation, a face-to-face meeting. I prefer an email as it is something you can keep referencing back to and it acts as proof of what the client has asked for. It is also useful if someone else is also working on the project as then they have access to the same information as you can.
The most common information you would find in a design brief is what the client expects from the project (such as a business card, logo design, poster, website ect) and useful information to help you reach that expectation (colours, images, font ect). Sometimes a client will have corporate guidelines that need to be followed and these bring certain restrictions like document size, company fonts and colours.