What is the general purpose for making or converting graphics / images to a raster format?
What happens to an image when it is rasterized?
Without knowing what kind of image you are rasterising, this is kind of a shot in the dark. Yet, here goes:
Rasterisation does exactly what the name suggests: making an image into a raster image, also known as a pixel image or bitmap. Rasterisation is usually done to vector graphics or images that have vector components. Vector components can be things like text objects that haven't yet been rasterised: the letterforms are vector images.
Examples of vector images are
.svg files. Examples of images that can contain vector elements are
.ps. InDesign files (
.indd) are also combined raster and vector elements, but
.indds are not meant to be distributed to a printer or somesuch: you'd export a
Rasterisation usually reduces the image to one flat layer, and thus limits editability to a minimum. You will want to keep a non-rasterised version of your file archived at all times, just to make adjustments later, if necessary.
Rasterization can mean different thing in different contexts:
It means to convert a vector graphic (a graphic made of lines and curves with infinite quality) into a pixel image, that you can only zoom in so much until you start to loose quality.
Raster formats: JPEG, EXIF, TIFF, RIF, GIF, BMP, PNG, and lots more
Vector formats: SVG (Scaleable Vector Graphic, most common), CGM, Gerber format (RS-274X)
If you are not using Illustrator or are not aware of what a vector illustration is, you most likely aren't using one. To check for sure, zoom in on the image and if the pixels become defined, you are looking at a raster image.