As it says on my favorite fortune cookie message ever:
"A diet is a selection of food that makes someone rich."
So let's look at a specific kind of diet.
Carb Cycling is an eating approach where you mix days with high carbohydrate intake with days where your carbohydrate intake is low. The idea is to have higher carb foods, especially starchy carbs (bread, sugar, pasta, etc.), on workout days when you body can make the most effective use of them.
One basic idea to diet is that after you work out or train hard, your body has used some or most of the available fuel held in your muscles. Your muscles hold fuel in the form of glycogen, which is a form of carbohydrate. While your body can make proteins, fats, or carbohydrates into glycogen, which you choose to intake can have a different effect on your body.
After you train hard, your muscles also need to be repaired and your body starts to need fuel to increase their size. Protein is required for this, but also it helps if your body sugar levels (insulin levels) are higher.
At the same time, excess calories from carbohydrates will be stored as body fat, not glycogen.
Carb cycling is an approach that attempts to manipulate your carb intake to most effectively recharge your glycogen and increase muscle growth on days when you train, while minimizing the effect of carbohydrates on off days.
That is a gross simplification of the entire process, but it's basically how it works - high carb on workout days so you restore glycogen and encourage muscle building, low carb on off days so you don't gain as much body fat.
Usually you see this diet approach in people attempting to bulk up, not lean out - it's an attempt to gain muscle optimally without gaining too much body fat. However, like all diet approaches, it's possible to tweak it to fat loss, too.
There are several approaches to this.
My personal favorite approach is the dead simple one in this article:
Fat-Burning Machine: Easy Carb Cycling For A Better Body (Scrawny to Brawny, like the book)
This article has the benefit of simplicity. Avoid starchy carbs and fruits and days when you don't lift. Have them on days when you do. Don't worry about specific macronutrient ratios. It favors a simple high carb/low carb split.
This article over on T-Nation is more precise:
Research-Approved Carb Cycling
It also includes links to most of the other good T-Nation articles about carb cycling.
Many of the plans include high, medium, and low carb days. The manipulation is more precise and nuanced.
Any way you do it, however, it boils down to:
Starchy carbs on workout days? Okay.
Starchy carbs on non-workout days? Not okay.
What are the downsides to carb cycling?
The usual social downsides are obvious - it can be hard to get people to avoid inadvertently sabotaging your diet. It's even more so when that diet varies day to day - pasta is okay on Mondays, but not Tuesday? Okay, fine - then you have to miss Monday's workout and swap it with Tuesday and explain to your family why you have to swap meals. That sounds like a joke, but it's a real problem - if many of your meals are shared, your family essentially has to make allowances for your diet.
This can also be a problem in terms of what foods to keep around - if you have leftover bread from yesterday, you can't eat it today unless it's a workout day.
Otherwise, it's a relatively simple approach that neither eliminates an entire category of food nor over-emphasizes it.
What counts as a workout day?
The other snag is that you have to earn your carbs. A few minutes on the exercise bike or a few sets of curls and tris after you skip squats and heavy sled drags isn't going to cut it. You have to work out hard for the carbs to be maximally useful and the body fat to be kept to a minimum. In general, if you aren't sure the workout justifies a high-carb day, or if you're be embarrassed to say it did, keep it a lower-carb day.
I can vouch for this working pretty well, but also for the complications of daily diet changes. When I had full control over all of my meals (I fed me or no one else would), it was easy. When my meals were partly under other people's control (come home to a cooked meal, for example) or I had a rotating workout schedule, it did not work very well. Too many days became inconsistent (high carb on non-workout days, or effectively low-carb all the time.) If you can nail down your schedule and not be tempted by leftover homemade pizza in the fridge or that tin of oatmeal you were allowed yesterday but not today, this can work very well.