When you're getting started as an affiliate, the statistics provided by the affiliate programs and networks will suffice.
I've been using redirected links for my affiliate links for as long as I can remember, so I can have shorter links, track my number of outbound clicks, and quickly change out the links to try other affiliate programs.
Sub-ID's are additional parameters you can add to affiliate links to better track which of your pages or sites are driving the traffic that is making you money.
Then there are ad servers, which enable you to plug in a short bit of code to places on your site(s), and then easily change out banners, as well as rotating a variety of banners.
Google Analytics is a free service from Google that provides lots of extra data about the performance of your site.
And finally, there are tools for A/B testing, so you can see which versions of a landing page are performing best.
While I said that the reporting from the affiliate programs and networks is sufficient for new affiliates, there is no reason to hold off on using the various options to enhance your data.