What a vector image is, how it differs from a pixel image, and when to use one.
Vector or pixel image?
A pixel (raster) image, such as a photo, is a fixed grid of coloured squares. Enlarge it past its size and it turns soft or blocky. A vector image is redrawn from its shapes at whatever size it is shown, so its edges stay sharp.
Vectors suit logos, icons, illustrations and anything printed at many sizes. Photos and detailed textures are better kept as pixel images.
Why SVG is the format to know
SVG is the vector format of the web. It opens in every modern browser and in vector design tools, the files are small, and they look sharp on any screen. Because an SVG is made of shapes, you can change a colour or move a part without redrawing anything.
Making vectors with Melon Cut
Melon Cut Design has a Vector chat. Vector graphic makes a real, editable SVG from a description, and Convert to vector traces a picture you upload into SVG. A new vector costs 10 credits and a conversion 3.