Lootcrates are available through a player killing mobs, or through voting for the server. The chance of a player receiving a crate from a slain mob is 1%, however this value is affected by the Looting enchant. The type of lootcrate received varies using the following probabilities:
Crate drop type (%)
 CommonUnusualUncommonRareMythic RareLegendary
Drop rate:79%10.5%5.3%3%1.7%0.5%

Contents of each crate are determined using another set of probabilities, which vary depending on the type of crate being opened. The differences between each crate can be seen here:
Crate reward chance (%)
 Common itemUnusual itemUncommon itemRare itemMythic Rare itemLegendary item
Common Loot Crate85%8%3.5%2.4%1%0.1%
Unusual Loot Crate42.5%37%10%7.5%2.6%0.4%
Uncommon Loot Crate21.2%40%24.8%9%4%1%
Rare Loot Crate10.6%33%26.4%20%7%3%
Mythic Rare Loot Crate5.3%17%23%28%18.7%8%
Legendary Loot Crate2.2%11.8%13.3%21%29%22.7%

Lootcrates of the same type can also be combined to increase their value! 4 lootcrates that are the same tier can be combined in a crafting table to create 1 lootbox of the next tier, for example:

4 Common lootcrates -> 1 Unusual lootcrate
4 Unusual lootcrates -> 1 Uncommon lootcrates
4 Uncommon lootcrates -> 1 Rare lootcrate
4 Rare lootcrates -> 1 Mythic Rare lootcrate
4 Mythic Rare lootcrates -> 1 Legendary lootcrate

Through crafting, 1 Legendary lootcrate equates to 1024 Common lootcrates.