I started using TrainingPeaks.com several years ago, but I didn't really start using it to its full potential until this year. For the last couple of years I kind of put a training plan together from bits and pieces of things that I had read without always understanding the big picture. It worked okay, but I never peaked properly for races. I would frequently put in the work, not always the right kind of work, but didn't get the recover part right going into my big races.
Two years ago I purchased a training plan for an event and then put in a lot of time and effort for the race. I ended up having one of those dream days and started to look at my training a little different. After that race I started to really dig into the data that the reports in TrainingPeaks was showing. The Performance Manager became my fitness bible after I spent some time getting comfortable with what it was telling me. The nutshell version is the blue line is your Chronic Train Load (CTL) or basically your fitness level. The pink line is your Acute Training Load (ATL) or how much work you have been doing. The yellow line is your Training Stress Balance (TSB) or how fatigued you are. Basically as you train your ATL increases which increases your CTL meaning that you are getting fitter. The side effect of this is that your TSB decreases, which means that your tank is starting to run empty. You can only push so long before you need to take some time and recover. What the image above is showing is the steady series of build and recover periods that I have done since I started this years training in December.
There is both a Basic Edition that is free and a Premium Edition that is paid for. I would highly recommend taking a look at the basic edition and if you like it, then spring for the premium edition. Both USA Cycling and USA Triathlon memberships offer discounts for TrainingPeaks.