The format is the full area where the final design will be laid out. In print design, the format is the page and in web design the format is the browser window.
Margins are the empty spaces between the edges of the format and the content. The size of the margins is what gives the content a general shape, usually a rectangle.
Flowlines are horizontal lines that separate the different sections of a grid into parallel bands. They help the reader follow the content of the layout. Flowlines also create stopping points, or edges for the elements to be placed on. Some flowlines are called hang lines and others are called baselines.
Modules are the building blocks of any grid. They are the spaces created between the flowlines and vertical lines. Vertical groups of modules together create columns. Horizontal groups create rows.
Groups of adjacent modules in vertical and horizontal areas create spatial zones or regions. A vertical region can hold a block of text, a horizontal region can hold a video. Regions can be organized proportionally or used to create overlapping zones.
Columns are vertical spatial zones or regions that fit fully from the top to the bottom margin.
Rows are horizontal spatial zones that fit fully from the left to right margin.