1. Niche - A specialization. So lets say you are into fishing and you build a site about it, your site would be within Sports/Outdoor activities category, however within that 'niche' you will find people that specialize in sea fishing, river fishing etc etc the more you specialize the more 'niche' your website will be, so if you talk exclusively about night fishing for cat fish in Mongolia you will be extremely niche but you will also likely be the most reliable source of information on that particular subject, however the potential audience will be smaller.
2. CPA - cost per acquisition. Go to wikipedia then add /wiki/cost_per_action
3. Affiliate - Go to wikipedia then add /wiki/affiliate_marketing