Please explain upsampling CDs

I see ads for CD players talking about upsampling CD to 24/192. Given that a CD only has 16 bits x 44.1 samples/sec of information on it, how can you possibly squeeze more out of it? Is this voodoo audio, or does it actually improve sound quality?

On a related note, I made the mistake of picking up the SACD of Wynton Marsalis' _Standard Time Vol 1_. After checking the liner notes I found that it was recorded digitally (probably in 16 bit) in 1987. I bought the CD used for $2 and compared. As far as I could tell, there was no difference in sound quality. Is this an example of inconsequential upsampling?

Thanks in advance for any info.

Here is a good link:
For your further edification read this one: