Would someone explain what "up sampling" means and how it can affect the sound?

I'm a "computer geek", so when I bought my current DAC, I was concerned as well when I read that it upsampled to high resolution DSD as part of its way of operating.

I concer with mapman: " Just listen to the DAC and don't worry about upsampling."

It’s just a change of the sample rate, any change of the sample rate is a lossy process and is audible especially on accurate setups.

Google the Grammy 5.1 guide, they mentioned it there as well.

I prefer NOS (sample rate unchanged), but modern DAC with 8x upsample sounds pretty damn good so there is no need to pay insane premium for NOS DACs unless you are made of money.

Well, I agree with chayro’s statement and disagree with gdhal's statements, "Seems to me it would be best to make an educated decision based on all possible information available. That would include specifications, your own subjective listening, opinions of reviewers, value of the component, etc." You don't listen to specifications and just because a reviewer raves over a unit doesn't mean you will like it. Bottom line is you buy what your ears like. Too many people buy for the wrong reason, and all you need to do is trust your ears.

I cannot tell you how many posts I've seen in which the person is looking for a DAC with a certain chip or a speaker with a first-order crossover, or a diamond tweeter or whatever.  IMO, it all doesn't mean a damn thing. It's how the product works as a whole that counts.  But then again, I truly believe that many audiophiles have no idea of what they're listening to anyway and just make purchase decisions based on reviews or pictures or whatever.  Of course, it's their money and their choices, so everyone is entitled to enjoy this hobby in their own way.  I just think that many audiophiles would be happier with their systems if they just listened more and read less.  

Oversampling - integer multiple of original rate
Upsampling - non-integer multiple of original rate

For instance, my Benchmark DAC1 is upsampling (and not the oversampling) DAC,