Your actual crit chance is a sum of many different sources. You get 5% baseline, another 5% from the Smuggler buff, up to another 5% from skills (depending on your spec), and then some from crit rating and willpower.

The amount you get from crit rating is on diminishing returns. This means that every point you get gives you a smaller boost to your crit chance than the last point did. This is based entirely and only on how much crit rating you have, though. Your crit chance can never be increased by more than 30% by __Crit Rating__ alone (though there's no hard limit on your actual crit chance).

Willpower works similarly. Each point gives you less of an increase to your crit chance than the previous point did, and similar to Crit Rating, this is based __only__ off how much Willpower you have. Again, like Crit Rating, your crit chance can never be increased by more than 30% by __Willpower__ alone.

So, for example, you could have 20% contribution to your crit chance from Crit Rating (which is less than the 30% cap from Crit Rating), and another 20% from Willpower (which is also less than the 30% cap on Willpower). Together with your 5% baseline crit chance and your 5% from the Smuggler buff, that gives you a 50% crit chance. Note that this final crit chance is above 30%. The 30% cap __only__ applies to the benefit you can receive from Willpower and Crit Rating, and applies to each __separately__.

The primary difference between Crit Rating and Willpower is how quickly their benefit falls off. Crit Rating gives far more crit chance per point initially (about 5-6 times as much, actually), but diminishes much more rapidly. Where it might take you 2500 Willpower to halve the amount of crit chance you receive per point of Willpower, it'll only take you about 450 Crit Rating to do the same per point of Crit Rating.

Make more sense?

If not, think of this visual analogy (I find visualizations often help me understand something):

You've got two large wide funnels, about 2 feet across at their top. You plug the bottom ends, then start filling them up. Because the funnels are much wider the farther up the water goes, it takes more and more water to increase the water level by the same amount (say an inch). DR works similarly, except the design of the funnel is that it would take an __infinite__ amount of water to fill the funnel entirely up to the brim.

Willpower and Crit Rating are in separate funnels next to each other. How much water you have in one doesn't affect how full the other one is (and vice versa), but your crit chance is equivalent to the total __height__ of the water in both funnels added together.