Internet traffic comes essentially from 3 sources, each has different charactersitics and you need to understand this.

For a brand new site on a new domain you only have two possible sources of traffic, type-in traffic and search engine traffic. 

Type in traffic is to a 'newbie' something a little difficult to get your head round, but basically it someone directly typing into the browser address bar a domain name with extension.  Why would happen, well for various reasons, firstly 'real world' advertising or promotion, secondly because it confusing similar to a brand name (and you won't want to get involved with that sort of thing would you), thirdly because it is a generic name where someone genuinely is typing the product or service name into their browser address bar.  How do you know if domain has type in traffic, well in the good old days i.e. 2 or 3 years ago Overture (now part of Yahoo) provided a useful tool at you provided search term results, there are sites out there have years worth of data cached, however as the data was extensively 'mined' almost all the domains with traffic are gone.  Google also do something similar today with the Google Adwords External tool, most of affialiate marketers use the tool to find searched for terms which are reasonably highly searched,  withdecent earning per click but with limited or little competition.  However the same tool can be used to look for domains with type-in traffic, in the same way as the old Overture tool.  Basically you have to understand user behaviour and we will talk a little more about it here The User, but in essence what the Google tool reports is the number of users that put the domain name plus extension into the google search box, what you also be certain off is that a percentage of users will also type the same thing into the address bar of the browser. I particularly like type-in domains as the traffic whilst lower in volume than from search engine is more reliable as less prone to Mr G. changing the rules. 

That neatly brings me to next area where traffic will come from that is search engines.  Again a whole section Search Engine Traffic and in fact we could probably spend our whole coming up with theories methodologies and ways to do this.  On this page all I'll say is it's about unique content and how competitive the area is, i.e. how many other sites are out there with content in area you are trying to publish in. If there is no competition then you will always be number 1 in Google for that particular search term, if there is compeition there are various techniques to help get backer ranking, and believe it or not 'Type-in' domains have benefit in that users of Google's Toolbar report back where they have been and this all goes into the melting pot to determing ranking.

Finally an area often not considered by affiliate marketers is to acquire a domain that use to have a site on it, thousands of domains get cancelled every day, most are junk but some have decent back links from places where normally an affilliate marketer you'd never get links from. All I'll say is from experience, the older the domain the better back links and sustainability of the traffic.  Some the tricks and techniques used to get a site ranked today using twitter, facebook etc. etc were never applied to these older sites and that is why the they work better and longer for you.  The backlinks and the PR or authority of those back links can dramatically affect ranking on site where there is competition.  Anyway as someone who has a large portfolio of UK domain names I'll share my experiences around here Domain Names