A Digivolution Stage is the form a digimon takes when they either Digivolve or Degenerate. These stages usually vary in power and are used to scale most digimon to each other.
There are at most 9 digivolution stages that a Digimon naturally goes through. These are Fresh, In-Training, Rookie, Champion, Ultimate, Armor, Mega and Ultra. Every digimon has a form they can go to to eventually reach at least Mega level.
Digivolution despite how the anime's portray and more of how games like Cyber Sleuth and Dusk/Dawn portray are not always linear.
This is not a canon Digivolution, but it is still somehow possible.
Now we Digimon fans have stopped trying to make sense of Digimon Digivolution lines even in canon. A big example of this is
Now by "canon" I mean the most well known "natural" Digivolution lines. However, according to one Novel it shows that a Digmon can digivolve into anything with enough data.
"Taichi’s blue device immediately glowed. Its gauge ascended until it broke past the critical point. Rising to a level high up that could not be physically seen in the Digital World, it became a dazzling light when it reached its summit, exploding into what appeared at a glance to look like a bulky, disordered mass of information. This flow of information was at a scale that could rival the galaxy, and pieces of it that reacted to the light were selected. G R E Y M O N Each of the small fragments of information glowed as they gathered together and flowed towards a specific point. The tumbling sequence of light became a helix, which rained down invisibly on the data that would equip Agumon. The information describing his form was rewritten in a flash, and it even called in for greater mass until, with a blaze of light that looked like flames, he transformed. “Agumon, evolve! Greymon!”
Anyway let's begin.
Fresh/Baby I/ Slime
These are the next forms of digimon. Now these forms have a little more protection than before. Most use "powerful" spit to damage attackers. Others can breath small flames or seeds. Others have thorns or strong fangs. However they are still very weak however, they can now at least train a little to digivolve faster.
Rookie/ChildThese are the forms most people are used to seeing. Rookies are usually the forms Digimon stay in when they have a Human Partner. These are also the forms they start really being able to fight seriously. This is also the time when they start fluctuating the powerscaling. Some Rookies such as Lucemon are able to fight higher stages on even grounds. However, most Rookies are comparable to each other. This is also the time most Digimon really begin to start living independant lives. Usually they had a stronger Digimon taking care of them. Now they can live on their own and train, or take part in jobs...Although sometimes they can still have stronger family members taking care of them.
Champion/AdultHere we are, the big guns. Once a Rookie trains enough or fulfills the requirements they reach the Champion level Stage. They are simply a more powerful form. Some Digimon have drastic forms though.
This stage is technically the last stage a Digimon reaches hence why it's the "Perfect" form. Most Digimon never reach Mega on their own without outside help. These Digimon are extremely powerful and some can keep pace with Megas.
Mega/UltimateThis is the Ultimate form. The form beyond Perfect (Cell wishes he made it to this level). This is technically the cap for Digivolution. The most iconic Digmon are Mega levels as well as the Royal Knights and Olympus Xlll. Most end series antagonists are megas. Mega Digimon can vary from Small Planet level to Multiverse level. This is technically the end all be all form that every Digimon who seek power claim to achieve.
ArmorArmor Digivolution is a very special Digivolution. This Digivolution requires a special Digi-Egg (For Example Digi-Egg of Miracles). When it comes to power they can range from Champion level power to Mega level power.
This is the form that usually surpasses Mega. These Digimon usually require the fusion of multiple Mega level Digimon. Or just a large number of Digimon in general. These Digimon are very powerful and they usually serve as the God Tiers of the Digimon Multiverse. Digimon like Alphamon is sometimes referred to as a Ultra as well.
Now this is a pretty much Season specific transformation. These are the Burst Modes. A little above Mega but not yet Ultra level. They are just large power boosts.
HybridsNow these guys are complicated. These are digivolutions related to spirits. Most notably the spirits of the Ten Warriors. These guys in turn become Susanoomon. This evolution is also called Spirit E
The Eater are beings introduced in Cyber Sleuth. They are beings lterally from a
Higher Dimension above the Digimon Multiverse. These beings absorb Data and then evolve accordingly.
This is a form of Digivolution only when a Digimon digivolves into an incredibly evil Digimon. This Digimon only yearns for Destruction and is usually a mindless monster who shows no mercy.
Some examples are
Families or "Species" are pretty much groups that Digimon belong to and there are 8 separate families.
Nature Spirits: These are pretty much Beast Digimon. These are simply animal Digimon. These Digimon are mostly Earth Elemental and are weak to Fire. Sorry Pokemon fans, but Fire beats Earth here.
Deep Saver: These are aquatic Digimon such as Fish, Sea Animals, Artic Animals and Water Beings of Water.....yeah.... They wield, you guessed it water. But for some reason they are weak to Steel......
Nightmare Soldiers: These are your Demonic Digimon, Eldrich Abominations, Cthulu Beasts and everything that is active in the night. These guys use the Darkness Element and are obviously weak to Light.
Wind Guardian: These are you Birds and other Avians that fly high in the sky. The utilize Wind and are weak to Thunder.
Metal Empire: These are your TRULY Mechanical Digimon as well as Mutant Digimon. Now I know a ton of Digimon look robotic, but half of them aren't classified as Machine Digimon. They are Steel Elemental and are weak to Water.
Virus Busters: These are your Holy Digimon. These Digimon are either Angelic or Holy Knight Digimon among other Divine Beasts. They are Light Elemental and are weak to Darkness.
Dragon's Roar: These are as the title explains Dragons, Reptiles or anything that can lazily be called a Dragon simply by being a reptile. These guys are mainly Fire Elemntal and are weak to Earth.
Jungle Troopers: Simply put, these are Bugs and Plants. Nothing more. They are as you guessed...That's right Thunder Elemental and in some games Plant. They are weak to Wind.
So this is simple to explain and for those planning on playing Cyber Sleuth this is the most important piece of info for you. These attributes pretty much tell you what kind of Digital Entitiy our awesome little monsters are. There are 4 types.
Vaccine: These Digimon are naturally made to fight Virus Digimon. Many Vaccines include Dragon and Holy Digimon. This is also very common with Avian and Aquatic Digimon as well. They are weak to Data Digimon.
Data: These Digimon are the Digimon that keep Vaccines from running amok. They are usually Beast, Plant and Machine Digimon. They are weak to Virus Digimon.
Virus: These Digimon are the Digimon that corrupt Data. These are usually Nightmare Soldier Digimon as well as Insectoid Digimon. They are weak to Vaccines.
Free: These are Digimon that are completely Neutral. These are mostly the Armor Digivolving Digimon/Descendants of an Ancient Race.
Variable: An odd Attribute for Hybrids. That is all.
That is all folks!!!. I hope you guys found this informative and fun. :)