CDs can hold a lot of data (approx. 650MB) that's probably enough for that book your going to publish ;)
DVDs can hold much more data (approx 4-8GB depending on type) and are better for video (movies) but can be used appropriately for storage of data as well.
The difference you will never see is how the data is encoded (burned) onto the discs.
So, if you wanted to make a soap-making documentary use a DVD. If you just want to store your computer files and pictures, a CD would probably be just fine.
As for downloading photos: If you want to keep adding photos to the same disc, then yes go with a re-writable. Depending on your PC, you may need to buy some CD burning software (Roxio, Nero, etc).
Best advice I can give you is to go to Best Buy and talk with them about what you have and what you want to do. They'll set you up fine.
Terminology: MB = megabyte. A measurement of data. I took the following from http://en.wikipedia.org/wiki/Megabyte to give you an example of what that means in real terms:
Depending on compression methods and file format, a megabyte of data can roughly hold:
* Roughly, a minute of near CD-quality MP3 compressed music (at 128 kbit/s)
* Approximately 100 pages of single-spaced 12 point font text in Microsoft Word. - translate that to approx. 65,000 pages (your book shouldn't be longer than that - will it????)
* Approximately 3 seconds of DVD-quality video
A digital photograph produced by a typical digital camera in 2005 might be 1–4 MB depending on the camera's image resolution and level of compression used.
- Approx. 160 pictures on average.
GB=Gigabyte (1,000 MB) From the same site:
# One gigabyte is roughly equal to 18 hours of MP3 music (at 128 kbit/s).
# One gigabyte is roughly equivalent to 11 hours, 40 minutes of Flash video (at 450x370).
Sorry for the techy stuff. I hope it isn't too foggy but I wanted to give you a foundation