A Digimon begins out as a Digi-Tama (Tama = Egg). From there, it hatches into a Baby Digimon, which is normally very weak with the notable exception of Arca Demon. Baby Digimon are usually unable to say anything other than their name, minus the -mon. From there, they evolve into Baby II, or In-Training in the dub.
Baby II Digimon are able to speak, but are still rather weak, although their fighting abilities are generally better than Baby Digimon, although they both are only capable of blowing bubbles. After a longer period of time than that of Baby Digimon into Baby II Digimon, they evolve into Child Digmon, or Rookie in the dub.
Child Digimon have attacks other than blowing out bubbles. They're usually not particularly adept at fighting, but are considerably better than Baby and Baby II Digimon. Child Digimon evolve into Adult Digimon after a rather long period of time, or, if a Digivice, D-3, or D-Arc is used, they can evolve anytime the device is activated.
Adult Digimon, or Champions in the dub, are usually considerably more powerful than Child Digimon, and are usually very adept at fighting. In general, they are also bigger and faster than Child Digimon, and with few exceptions have far better attacks than the Digimon before them. Adult Digimon evolve into Perfect level Digimon after an extremely long period of time, or with the help of a Digivice, Tag and Crest, or a D-Arc and Blue Card.
Perfect Digimon, or Ultimate in the dub, are generally vastly superior in all ways to the levels below them. They have far more powerful and varied attacks, and were once considered fully evolved, before Vamdemon evolved into Venom Vamdemon in Digimon Adventure. Perfect Digimon can evolve into Final level Digimon, but only is a special event occurs, or with Matrix Evolution.
Final/Ultimate level Digimon, or Megas in the dub, are the most powerful level. They are completely and utterly superior to all other levels, and only very special or gifted lower level Digimon can even survive against them, such as Lucemon, Arca Demon (Baby, Baby II, Child, Adult, and Perfect.), and Skull Satamon. Final Digimon are very rare, and only occur through special events, or through Matrix Evolution. Final Digimon can Mode Change, and Jogress together, but they do not advance in level when they do so.
The above six levels are normal ones. The following are levels that are outside of the normal evolution lines.
Armor Digimon: A Digimon, usually a Child, who has used a Digimental and evolved. Most Armor Digimon are around Adult level, but a few near Perfect and possibly even Final in power. No Armor Digimon is stronger than Final, however.
Hybrid Digimon: A Digimon who has used one of the Spirits created by the Warrior Ten to evolve. There are Beast, Human, ???, Advance,. and Z Hybrids. Beast and Human Digimon are roughly strong Adults, Advance Digimon are roughly around Perfect, and Z Hybrids are around strong Perfect/weak Final in strength. The only ??? Hybrid was a fusion of a Human Hybrid and the Digi-Code of a Final; it was weaker than an Advance Hybrid.
Methods of Evolution
Jogress: Two Digimon, usually of the same level (The only exception I can think of is Chimeramon + Mugen Dramon = Millenniumon, but he was an accident.), combine to evolve into the next level - or, if they are Final level, only to become stronger, but NOT to advance in level.
Fusion: Two or more Digimon fuse together, sometimes evolving to the next level, sometimes only becoming stronger.
Mode Changing: A Digimon that Mode Changes DOES NOT advance to the next level, although they do become significantly stronger. Mode Changing is often not possible without another Digimon giving power. For example, Omegamon gives energy to Imperial Dramon Fighter Mode so he can Mode Change into Imperial Dramon Paladin Mode.
Matrix Evolution: A Digimon and their Tamer combine, the result being a Final level Digimon.
Warp Evolution: A Digimon warps through several levels to attain Final.
Spirit Evolution: A Human or Digimon uses a Spirit to evolve into a Hybrid.
Double Spirit Evolution: A Human or Digimon uses both the Human and Beast Spirits of the same element simultaneously, resulting in an Advance Hyrbid.
Hyper Spirit Evolution: A Human or Digimon uses ten Spirits simultaneously to attain Z-Hybrid.
Normal Evolution: A Digimon simply glows and evolves to the next level, or in some cases skipping one, and it is sometimes aided with a Digivice, Tag, Crest, or other item.
Ancient Spirit Evolution: All the Spirits at once used by any number of beings.
Please note this section is theoretical and out-of-date with the current views of the webmaster.
Almost all Digimon differ in strength, even from others of their kind. One reason may be because of how they evolved [experience being another, probably more important, factor.]If so, for any given Digimon, there cannot be more than four or so "levels" of power. I will use Cyber Dramon for this example. There can be strong, average, or weak Cyber Dramon [please take into account this is referring to the base power of newly evolved Cyber Dramon, not all Cyber Dramon ever.]], depending from who and how they evolved to Cyber Dramon.
If a Lighdramon has a win ratio of 40% [meaning it wins 40% of the time], it may evolve to Cyber Dramon. However, it would be a weaker than normal Cyber Dramon - the reason being Lighdramon's win ratio.
If a V-Dramon has a win ratio of 60%, it may also evolve to Cyberdramon. However, it would be an average-powered Cyber Dramon, compared with all the possible Cyber Dramon.
A Revolmon and Greymon may jogress to Cyber Dramon. The result would be an above-average Cyber Dramon.
Put simply, the higher the win ratio, the stronger the Digimon. Jogress Digimon are stronger than win ratio Digimon. If the win ratio of a Digimon is below 40%, I would suppose it ends up devolving, like Andromon in Digimon Tamers.