What is affiliate marketing?
Think of a "merchant" who sells a product. This trader has an advertising campaign on television, in newspapers or on the Internet but want to sell more. So, the company make an affiliate program where anyone can join to create their own ads.Every time a sale is made through these redirections the affiliate (publisher) gets a commission. Simple, right?
So once you've joined the affiliate program you receive your own link which will register all the commissions generated by you.

What is an affiliate network?
Affiliation is made with the trader, but with the increasing number of companies offering affiliate programs to choose the best product to promote is more complicated. Here comes into play affiliate networks.An affiliate network is a company that deals with the relationship between affiliate and merchant. Affiliate Network are registered on the site where you can find different offers that you can promote and can see exactly what is the commission received from a sale.


Methods of promotion
There are hundreds of ways to promote the offers but I will only rank them in two categories: promotion paid and unpaid promotion.

Paid promotion:
Among the paid promotion methods I will list some most important: promotion through PPC (pay per click), promotional email (legal messages sent, not spam) or buying advertising space and links on other sites. These methods can generate very much traffic to your site but also are riskier than methods unpaid because you're not sure if and how much you profit.

Unpaid promotion:                                                                                                           Methods to promote are endless and only imagination is the limit. I will only review the most common methods such as SEO (search engine optimization), social bookmarking (eg del.icio.us), viral marketing (eg marketing youtube), social networking (eg Twitter.com ). To apply these methods do not need any money (in theory), but instead requires a higher workload than in the paid methods. Traffic volume generated by these methods is generally lower, but can be profitable in the long term in some cases (eg SEO). These methods are helpful to beginners who have no experience or money to invest in other methods.

Targeting
The most important thing when you choose a product is to be more targeted. That is the basic rule. Have a site seeks to promote poetry books with poems or something related to poetry. If you have a blog about mobile phones then is indicate to promote phones. Do not promote random products because you lose your time. What's the chance that a visitor enters your site to read the latest model nokia Reviews and wants to buy some books of tales, for example? So, targeting is a very important factor in your way to success in affiliate marketing.


How do you choose what to promote?
Once you have established your target audience, it's easy to choose what to promote. See what works best in your niche. Ask the world, join forums and make a list of 3-4 products you think will be successful for your site.
If you still fail to decide then make accounts on  more affiliation networks and choose some products to promote.In the end the best product for your site will be noted in the shortest time.