Basic:
Egg->Fresh->In-Training->Rookie

From Rookie, you can go along two lines; the Natural line or the Aided line.

The Natural line follows the Digimon's natural abilities and the levels it could achieve without human intervention. They have higher base stats but their abilities are blocked in certain situations.
Rookie->Champion
The Champion level here then can digivolve into two different Ultimates - one with the Yin crest and one with the Yang crest for their partner. Each of those Ultimates has its own Mega it can arrive at.

The Aided line implies human intervention; these forms are slightly weaker, but allows for more intervention and options from their partner. Generally, these forms do NOT work in the Real World.
Digimentals are used to arrive at the Armor level, equivalent to Champion; generally these bear the same two crests that the partner does, but with time and effort, more can be unlocked and used, though doing too many may block a Digimon's ability to Naturally digivolve.
These Armor levels can DNA digivolve with another Digimon to form a Fusion Digimon, equivalent to Ultimate level; different results can be achieved by fusing with different Digimon, but the true strength of this level can only be unlocked if there's a strong bond between the two Tamers.
The strongest form from the Aided line is the Biomerge Mega; it comes not from the Fusion level, but straight from the Rookie level, but only after a Digimon has experienced both Digimental and Crest evolution from the same Crest at least once, and only after their Partner is bonded enough with one or both of their Crests to allow them to combine with their Digimon in this way.


(A chart and less confusing descriptions will be posted soon-ish, as will point requirements, if such things are decided on)