Tar is used to explore common Crypts. By exploring such Crypts, you can get materials for crafting standard equipment. The higher the level of a Crypt, the more Tar you need to spend in order to completely explore it, and the more materials you will get once you have done so. Exploring different Crypts grants you the possibility of obtaining two specific materials instead of one: the particular material depends on the Crypt you explored. However, bear in mind that this chance is never 100%!
Rare Tar can be used to explore rare Crypts. Exploring such Crypts allows you to obtain gems, which can be socketed into crafted equipment. Unlike common Crypts, rare ones don't offer you the chance to get two specific gems instead of one.
Please note that in order to explore rare Crypts, you need not just to have conducted the relevant research in the Academy, but you also need a specific key to open the Crypt. You can get a key of the required level by defeating Elven Citadels of the same level.
Epic Tar is used to explore epic Crypts. Exploring such Crypts allows you to obtain materials for crafting unique equipment. Much like common Crypts, various epic Crypts grant you the possibility of obtaining two specific materials instead of one: the particular material depends on the Crypt you explore. Epic Crypts also require a lot more Tar than common or rare ones. In order to explore epic Crypts, you first need to conduct the level 3 Crypt research in the Academy.
The base maximum march time to a Crypt is limited to 20 minutes. Various technologies and Hero talents allow you to increase this time, however. Also, please note that the Ayrin avatar removes all such restrictions immediately.