Vector images consist of lines and curves. In general they are shapes with outline and fill and with that you compose everything else. Because you specify shapes in terms of mathematical equations they scale well and look the same in every resolution or size.

Raster images on the other hand consist of a regular grid of pixels each of which has a color. Changing the size of the image degrades its quality as you only have the original pixel grid as a starting point.

Converting from raster to vector basically means you'd have to try to find out what shapes can be used to approximate the look that was achieved with the raster image. For straight lines this is usually straightforward, but for curves and similar the process gets a little more complicated to get the look right.

Also it gets much harder if the raster image only has low resolution.

Vector images consist of lines and curves. In general they are shapes with outline and fill and with that you compose everything else. Because you specify shapes in terms of mathematical equations they scale well and look the same in every resolution or size.

Raster images on the other hand consist of a regular grid of pixels each of which has a color. Changing the size of the image degrades its quality as you only have the original pixel grid as a starting point.

Converting from raster to vector basically means you'd have to try to find out what shapes can be used to approximate the look that was achieved with the raster image. For straight lines this is usually straightforward, but for curves and similar the process gets a little more complicated to get the look right.

Also it gets much harder if the raster image only has low resolution, such as on the web.