The quick answer: A rasterised (or rasterized in American English) image is an image composed of pixels (each containing a colour), this then has the expectation that there are so many small pixels that it begins to resemble an image. Whereas a vector graphic uses mathematical equations to draw lines and shapes, similar to plotting a graph. Generally referred to as paths, these are not mapped to pixels which mean that they can be enlarged and reshaped without quality loss.
To go more in depth, both have their pros and cons:
Rasterised (or rasterized) image file types include .jpegs, .png, .tiffs, .psd and many more (we can go more into the differences in file types in the future). The pros of rasterised images often come from the reliability and the data (mainly colours) that can be stored, this is why photographs are stored as the aforementioned file types. It also allows for more complex photo effects and allows image editing to be much easier.
The cons generally come from the fact that each part of an image is mapped to a pixel, this is why low-quality images can often look blurry or blocky. This can, of course, be avoided by making sure that the image is the correct size/resolution for what is required, for print this is usually 300 pixels per inch (this means that a two-inch square image will be 600×600 pixels wide). Also to take into account is that once an image has been made smaller, it is impossible to restore the data, which is why it is generally best to keep one large “master” image before resizing a copy. And finally, another con is that the file sizes of rasterised images can be very large depending on the content, and lossy compression algorithms (such as what is used in jpegs) can degrade image quality even further.
Rasterised images are usual produced and worked on within photoshop (other cheaper/free alternatives are available).
A vector image is usually stored within .pdf, .eps, .svg and .ai filetypes. The pros of vector images lie within the quality and versatility of how data is stored. As shapes, lines and text are not mapped to pixels but instead to equations, this means that they almost have an infinite resolution and can be enlarged and shrunk without any quality loss. This means that they are perfect for logos which may have to on anything from business cards to A0 posters. Most vector file types also allow you to store rasterised images alongside vector artwork, this means you can have your high-resolution photographs alongside perfect quality text (which will be how most documents will be designed for print).
And also vector graphics can be rasterised into images at whatever resolution you require.
A pdf is generally considered the best way to send artwork and has become the standard way to send print files.
The cons are generally (opposite to rasterised images) is in the data that is stored, an example is that a vector image can struggle to store complex gradients (which is why they can’t be used for photographs).
Vector images are usually designed in Adobe Illustrator or in Indesign.
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