What are different keywords?

[Sunfrog]

So, I was adding keywords to my site's pages and I read somewhere that they should be different for each page. What does different mean? Like, are these different?

cow, brown, milk, dairy ---- heifer, milk, bovine, dairy cow

Are this different or no? They both have cow, milk, and dairy but slightly different. Would milk get kicked out? It's the same on both pages. What if I need milk all over the place because I sell milk on my site?

[Submerge]

Match specific keywords for each page.
Like if you have a site covering PPC and sub pages for Adsense and YPN it is like

Main page: PPC, Adsense, YPN
Adsense page: Adsense, PPC
YPN page: PPC, YPN

[Sunfrog]

So it's okay if some words are the same? Would this be okay?

Main page: PPC, Adsense, YPN
Adsense page: YPN, PPC, Adsense
YPN page: Adsense, YPN, PPC

P.S. How many keywords should I have for each page? Is there a limit?

[boron]

It is just good if you build the text on all pages naturaly. You will use keywords as you'll find appropriate. Exagerating with them - like mention milk 100 times on a page with 300 words is an obvious spam and Google will check this. But having milk 100 times in a page with 3,000 words and many links is nothing special.
Places important for keywords (beside in the page text) are: Site title and subtitle, domain, anchor text on backlinks and on site internal and external links.

Keyword phrases are important. So not only milk, cow.. but cow milk, goat milk, milk fats, milk composition, "nutritions in milk"... Type "milk" in this Keyword suggestion tool (http://tools.seobook.com/general/keyword/) and you'll see exactly what are people looking for when they think on milk...

[Submerge]

I would say to limit your keywords to 20 a page at most. And I wouldn't add YPN to the adsense page, as I think a bot doesn't look for order.

[boron]

Google is changing its ranking algorhytms constantly (especialy recently). Google is trying to be set up this way that the most relevant - naturaly grown and content (not marketing) sites will be on the top. Yahoo on the contrary put a lot of weight to marketing oriented sites. This came so far that Yahoo itself launched a Yahoo mindset tool with which you can filter results according to their "marketing level" as shown in this example (http://mindset.research.yahoo.com/search.php?p=car).