Grids are used by designers as a starting place to more easily develop a coherent layout because they create a system of organized space to work with to ensure consistency. They are used by designers because it is a way for the space to be controlled in a way that is flexible enough to allows for creativity and at the same time make a layout coherent and whole. Grids create a system that allows for designs to be structured in a way that is visually appealing for a viewer to comprehend.

Modular Grids are more specific in that they have uniform horizontal and vertical sections from top to bottom and left to right. By using this type of grid, it is possible to create a hierarchy strictly based on placement within the grid, even though adding other elements would contribute to this as well.

Margins are invisible vertical spaces between colums or around the edge in a layout.
Columns are vertical blocks in a layout which are seperated between margins. They are used to assist with readability on a page.
Grid Modules are the smallest area of a grid that contains information.
Flowlines are lines that go in between characters in a grid.
The gutter is the space that appears in between modules.
A type family refers to fonts that are of a similar form and style.
Type styles consist of different weights, widths and heights. These may include regular, bold, italic, condensed and extended.
Hirearchies are used essentially as a guide for viewers to more easily indicate the importance of certain elements used in a design based on placement and use in a layout. They are essential in design to set different aspects of a design apart in a meaningful and purposeful way.