The two box types are Collector's Cache and Build Your Own (BYO). You can also get the I Want It All collection (IWIA) and now both the Collector's Cache and IWIA have subscription options for three or six months. Collectible Geek differs from other subscription boxes in that it mainly is not subscription-based, but open by theme for the following month.
Here's a general breakdown of the system using January, February, and March for the example.
Mid-January: March's theme is announced (March will be Totally '80s).
February 1: All three March box options go on sale for anyone not already subscribing. When they sell out, they sell out (and they do within a week or so).
March 15: Boxes ship.
Since you're buying about six weeks in advance with Collectible Geek, this is a really great option. You can also email them to skip a month if you don't like the theme.
Here are the details on the box types:
Collector's Cache: Minimum of five collectible geek items, including exclusive prints and shirts, trinkets and toys, and Funko products.
BYO: Base collection includes two Funko Pops. You can then add a third, or some Mystery Minis, ReAction/Legacy figures, or a Wacky Wobbler. These are related to the Collector's Cache theme, but usually a bit more specific. For example, February's Collector's Cache is Fantasy, while the BYO is Game of Thrones.
IWIA: You get BOTH the Collector's Cache and fully upgraded BYO in one shipment. The fully upgraded BYO includes 3 Funko Pops, a ReAction or Legacy figure, and a Wacky Wobbler.
March's BYO theme is '80s Cinema, so get your subscription or set an alarm for February 1! :)