Not a Comprehensive Guide.

iPod Classic

Hyperlinks to specific models:

iPod Classic, iPod mini, iPod nano

This is the main full-sized line of iPods that debuted in 2001 and got its last iteration in 2009.

1st Generation [OG] (2001)

The wheel mechanically spins. FireWire 400 Only, NO USB or 30pin connector. Came in 5GB and 10GB, no real mods exist for this model because of how incapable it is to run more storage. Throw on a 3rd Gen hard drive, that's about it. Cool collector's piece.

2nd Generation [Touch Wheel] (2002)

Same design everything except the control wheel is touch-sensitive and doesn't spin. FireWire 400 Only, NO USB or 30pin connector. Came in 10GB, 15GB & 20GB. This generation is also a collector's favorite, not many mods available but on the higher-end in price for a working unit.

3rd Generation (2003)

Every button is touch sensitive and storage is flash-mod upgradable to 128GB. USB Sync is now supported but you will still need FireWire for everything else, such as drive restoration and charging the battery. Looks really cool in the dark or with a red backlight mod (like an m:robe) for a nighttime car aux device.

4th Generation [Mono/Color/Photo] (2004)

The first iPod with the standard click wheel design. Storage is flash-mod upgradeable to 256GB. USB is finally fully supported along with FireWire. This design is the iconic white click wheel debut for full-sized iPods, featuring a monochrome display model, and 2 color screen variants before it was discontinued. Deepest clicks on any click wheel. [satisfying as hell]. iPod Photo pictured.

5th Generation [Video] (2005)

As the name implies, this model introduces the video playback feature. Take your DVD's (or files) with you wherever you are on a pretty good screen for the form factor. Album art is not displayed on the main menu but rather a generic menu. The 60GB and 80GB models also have 64MB RAM onboard, meaning those are the earliest models you can flash mod your iPod up to 1 terabyte (TB).

6th Generation ['Original' Classic] (2006)

The name of this model is really weird but it splits in meaning, "Original" is meant to differentiate the next 2 models of Apple's renaming of this line of products as the 'iPod classic' after the nano & shuffle launched in 2005.

It does comes with the best operating system that Apple gave the iPod, with its main limitation being the iPod locked to the 80GB and 160GB hard drives and up to 128GB of flash storage. A great option for people with smaller libraries and is a perfect companion and inexpensive option. Custom front and back cases are available with custom hard drive/sd card storage label. This model also introduced the aluminum front plates.

6.5 Generation [120GB] (2009)

7th Generation [160GB] (2009)

A hardware revision for the 6th gen was released by Apple to replace all of the 2006 models. While the "6.5" and 7th gen models were released at the same time, the 6.5 gen models are the 120GB versions of this generation that retain the same 128GB flash storage limitation that its predecessor has. All 120GB iPods have this limit. The main way to differentiate this model from the 6th is the color; 6th: Black, 6.5 & 7th: Space Gray.

The most valuable board and iPod currently available, this model released at the same time as the 6.5 gen iPod but this model is the only aluminum front plated iPod without the flash storage limitation. This makes the 5/5.5 (80GB) & 7th generations favorites for people with massive libraries as these can detect up to 2TB of SD cards however, performance takes a hit and breaking the iTunes song limit (50,000 songs) will have you running into issues. (500GB-1TB suggested)

You do not need the model number to identify all of these different generations, they are all cosmetically different.

What separates this model from every other generation, is the storage size badge on the back of the iPod. It is the smallest of all the iPods and it is very obvious in person and in pictures (if you happen to find one of these in the wild for cheap). Separating this revision from the original 2006 is the color. 6th: Black, 7th: Space Grey.

Note for aluminum front plate iPods:

Apple officially states there are only 6 generations to the iPod classic (essentially grouping all 'aluminum' iPods together), which means the last next 2 generations listed are unofficially named. But because they vary in release date, revisions and most importantly features, so they will be separated. You will likely encounter these unofficial names on sites like eBay as well. Differences are notable.

5.5 Generation [Search] (2006)

Cosmetically and functionally identical to the 5th gen, models sold after September 2006 have an added search feature to search all content on your iPod. Because of this feature, it tends to be a popular option for those looking to modify their iPod because it is the first iPod to be able to handle 1 terabyte of storage.