How in the world do I read these model numbers?

Dell Optiplex Desktops:

First number:

This is the class of the desktop. For example, a 300 / 3000 series desktop will have less slots on the inside for upgrades and maybe less USB ports as compared to a 700 / 7000 or 900 / 9000 desktop. Unlike the laptops, the build materials between classes are the same.

Second or Second / Third numbers:

If the model number has 3 digits, the second number signifies the generation of the desktop. If it has 4 digits, the second and third numbers signify the generation of the desktop. A 4 digit model number is newer than a 3 digit. Each generation would be kind of like a model year newer car. Each year, the outside of the car is refreshed a little, and the inside generally gets a bump in speed and power. Same with these! Going from one generation to the next will generally see a bump in looks and performance.

Last number:

They all end with a 0, nothing special to report here, captain.

Example: An Optiplex 390 would be a model step down (less ports and upgrade options), but a model year newer (so potentially faster) than an Optiplex 980.

Dell Latitude Laptops: Very similar to the desktops, but with an added number for the screen size.

E at the beginning - in front of every model number. Rumor has it that no one really knows what this means.

First number:

This is the class of the laptop. The 5000 series, while good, has a slightly lesser build quality than the 6000. Whereas the 5000 series is generally made of more plastic, the 6000 is more metal. The innards of the laptop are largely the same.

Second number:

This is the screen size of the laptop. A 4 means the laptop has a 14.1" screen, whereas a 5 means it has a 15.6" screen. For example, a Latitude E6420 and E6520 are essentially identical with the exception that the E6520 is a little bigger to accommodate the larger 15.6" screen.

Third number:

This is the generation of the laptop. A 0 in this spot signifies that it is the first generation. A 3 would signify that it is the fourth generation. Going from one generation to the next will generally see a slightly thinner, faster laptop.

Fourth number:

They all end with a 0, nothing special to report here, captain.

Example: A Latitude E6520 compared to an E5430 would have better build quality, a bigger screen, but would be a year older, so potentially slower than the E5430.