They will handle all details as far as tracking your associates, tracking the sales, handling the money... They will remunarate you a
percentage of each sale (up to 30% of the money you are paying out to your associate partners) for organizing all these activities and keep track of it. You have to pay
especially a set up fee.
Some of mentioned services offer all these services with a cheap set up fee, but their percentage per sale is high (i.e. ClickTrade). Some of that services
requires high set up fee, but in that case, percentage per sale is lower (i.e. LinkShare). If you use some of these services, your profit will be lower!
It seems at first that there are low set-up costs, but if you do your Associate Program in the best way, this could be translated into hundreds and often thousands
of dollars in fees you have to pay monthly. One of the benefits is that many associates will use your program even if they didn't saw your site before. You can also choose sites which
are acceptable as your associates by your opinion. In that case, you will get a lot of offers for your associate program, but they can't become your partners, before your approval.
This is good choice if you don't have enough money for purchasing your own associate program software. Of course, this is better method then "miror pages", but
it requires money, not a lot of time or knowledge of HTML. But, you must be careful about prices of these services. You must not permit to pay for these services more then you can earn
from sales. Check all of mentioned services, their prices, percentages per sale, terms and conditions, and then choose the best service for you.