I'm trying to book my AVE tickets online through renfe, but I'm having some problems:
A) What is a N.I.F.? It asks for it, but I don't know what it is...
B) It won't let me input my six-digit postal code. What do I do?

Can someone please help me?

NIF is the acronym for our Spanish Identity Card Number.

Of course, as I am Spanish, I didn't have problem filling those fields. Maybe someone who actually had them (many foreign forumers have purchased their tickets through renfe.es) can help you out telling you what they did. However, in case none else responds, I would suggest you to put your passport number in the NIF field, and to shorten your 6-digit postcode to 5 digits (the standard in Spain) eliminating the first or the last digit, at your will.

The NIF field is for identification purposes, and your passport can do, and the postcode is only statistical, I guess.

try this website

http://horarios.renfe.es/hir/ingles.html

I think I put in my passport number. You should have no problem doing this. Years ago, I purchased RENFE tickets online and then picked them up when I arrived at the airport with a confirmation code. Good luck!

Tried my passport number. Didn't work. It told me that the 'N.I.F. is incorrect'.

I think I might try to book through Eurail and simply tell them not to send the tickets to me.

The Renfe website is a shame. Absolutely chaotic and stupid.
I can tell you from experience, trust me. I am an IT consultant and I worked for them. Renfe is really a chaotic company, so the site reflects this perfectly. Sure, the AVE is magnificent and efficient...but is nearly the only efficient thing they have.
How can they request a NIF? a NIF is an ONLY SPANISH number used for fiscal purposes, not for identification only. Nobody outside Spain has a NIF. Nonsense.

I am testing the RENFE Website and, at least in the English version, it don't do any reference to NIF, but to an ID Card (field that I filled with 12345678 and worked fine). The only 'weird' thing for a foreigner is that a second surname is compulsory (rule valid for Spaniards, but not necessarily for the rest of the potential customers). In that field, put your mom's maiden name or whatever you want.

I cannot do further tests, because at this point I would have to enter a valid credit card and am not willing to do it, but it seems that they have different versions of the registration form by language. How did you try to buy your tickets, Carlotta? Through the English page (available on top right of the page Tim Hortons Man provides in this thread) or the Spanish one?

I tried to do it through the English website. Do the pages ask for different things? Seems odd...

Edit: Just went back again, and tried it with an eight digit number in the NIF field. It worked. Turns out my passport number wasn't long enough.

Is a 'plaza' a seat? Sorry if I'm being a bother.

It is not the literal meaning but, yes, in this case "plaza" stands for "seat".

Feel free to ask.

I had the same problem a few days ago and finally figured it out. You should enter your passport number, but the site will not accept the entire number. I can't remember exactly how many digits it will accept, but I think it's all but the last 2. Try it and see. I just kept trying until it worked, so I can't remember for sure. I agree that the site sucks.