You can't save a jpeg as a RAW file no, although you're not really comparing like with like here. RAW files are the well, 'raw' data from the sensor before its changed into an image file. Its not a standard file type like jpeg or TIFF and is specific to the camera model (DNG files are a little different in that they do introduce standardisation to RAW although not many camera manufacturers use it at present). Its best to think of you're RAW files as your 'digital negatives' ie they are the starting point from which you derive all of your images from. It can't actually be altered, if you change settings in a RAW converter, those changes are stored in a separate 'sidecar' file.
Jpeg and Tiff are, as I say, standard image formats that can be read by pretty much any software. TIFFs are saved with either no compression at all or 'lossless' compression. Advantage is that the file remains the same despite repeated saves and resaves, disadvantage is much bigger file sizes. Jpeg uses a form of compression that loses data each time the image is saved. You can alter the level of compression used in camera or in image software. Advantage is smaller file sizes and as a result more practical for the web, disadvantage is the losses of data if you repeatedly resave, also the fact that file sizes are not uniform and vary depending on the content of the image ie more detailed images bigger files.
Its generally accepted that resaving jpegs is a bad idea because of losses building on losses. If you save an image which you think you wil want to change it is best to save as a TIFF (or, if you are using Photoshop you could use PSD which is also lossless). Don't forget its only the resaving process that causes degradation in jpegs, just opening and closing without making any changes causes no problems. If you find that you have a jpeg which you simply want to have another go at re-editing you'd be best advised to start from scratch again with the RAW file, as such I'd always strongly advise that you keep your RAWs. You can save a jpeg as a TIFF in imaging software but any losses that have already occurred will remain in the image, it just won't get any worse.
The generally accepted thinking is to have a 'workflow' depending on what you want to use your images for. So some people keep several versions of their image eg a PSD file with all the layers intact, maybe a 16 bit TIFF for printing, a high quality (ie low compression high resolution) jpeg for general use and a low res jpeg for the web. And keeping the RAW file
. Its an individual thing and you may not need to keep all those formats, you may not have the storage space for a start (although buying extra is very cheap these days).