Vectorization.org is a free online raster to vector converter that can convert raster images to vector images. It works with Black & White images only. This free vectorization software is completely online. You just need to upload your raster image, and it will let you download vector version of that. If you want a more advanced raster to vector converter, you can try RasterVect.
A raster image is made of pixels. So, when you try to blow it up, it starts losing its sharpness. On the other hand, vector images stores image as a collection of lines, curves, and other geometrical shapes. So, it can be easily blown up. You can edit a vector image with a free vector graphics editor.
If you have a raster image, and want to convert it to a scalar format, Vectorization.org is a good choice. You can upload file in JPG, TIFF, or PNG format. Then you can convert it to SVG, EPS, PDF, Postscript, Gimppath and XFig format.
You can upload a file up to 5MB in size.
Check out Vectorization.org.