A graticule is a network of lines which can be used for geographic plotting, scale, and focusing, depending on the application. While sometimes described as a grid, a graticule is not necessarily a grid, and there are several different ways to describe coordinates within the graticule. A common example of a graticule is a grid of lines on a map which corresponds to longitude and latitude.
A geographic graticule is a network of geographic lines. People locate objects within the network of intercrossing lines by looking specifically at their geographic locations. Rather than giving locations in terms of the grid, such as with the use of numbers along the X and Y axis, people talk about their latitude and longitude. A geographic graticule may use curved lines for longitude to more accurately depict locations within the network of lines, since the globe is round and the graticule is flat, requiring some adjustments in the interest of accuracy.
Graticules can also be attached to eyepieces. An eyepiece graticule can be used to superimpose a grid on whatever someone is viewing through the eyepiece, whether it's a colony of microscopic organisms on a petri dish, or the natural environment as viewed through a spotting scope. The lines of the grid can be used to provide a sense of scale, as for example when people want to measure objects in their view, and also for navigational assistance, allowing people to isolate objects within specific squares.
Having a grid of crossed lines can be very helpful for some types of optical focusing. It can also be beneficial when people are aiming, and when people want to provide absolute information about what they are viewing. Rather than saying “look to the upper left of the image,” with a graticule, someone can say “check square F2.” Graticules can also be used for measurement and accurate population counts; it is easier to count the contents of each square and add them up than it is to try and count everything in the image at once.
Different graticules are designed for different purposes. An eyepiece graticule may be designed to provide information about scale, or to act as a simple grid for the purpose of isolating objects within the grid. Map graticules are carefully calibrated so that they correspond as closely as possible to latitude and longitude in the real world. Even with adjustment, at high latitudes, it can be difficult to be accurate.