D-EXP (Digimon Experience Points) is the level at which your digimon can digivolve. For example:

In-Training and Rookie:1.0 (You start off with 1.0 -usually-)
Champion:2.0
DNA Digivolve(To ultimate):2.5
Ultimate:3.0
DNA Digivolve(To Mega):3.5
Mega:4.0
Bio-Merge:5.0
Jogress Evolution:6.0

To gain D-EXP, you must train and battle. At the end of the topic/post you will receive a designated amount of D-EXP determined by how well you RP.
E.g:

Agumon's D-EXP:1.0
(Agumon defeats an opponent in a fight with an average Roleplay)
Agumon gains 0.1
Agumon's total D-EXP:1.1
(Agumon needs 0.9 more D-EXP to get access to his champion level)

Explanation of DNA Digivolution and Jogress evolution:

DNA Digivolution:
DNA Digivolution is the process in which to digimon fuse into another, that is obviously stronger. For example, two champions will create an ultimate. Two ultimates will create a mega.
You can DNA digivolve with an allie or you may later earn another digimon partner too digivolve with. If you're posting as a DNA digivolution that you're sharing with another member, you need to take your posts in turn.
E.G:
Billy and Stanley both control Paildramon.
Billy goes first.
The opponent goes next.
And then Stanley does his post.

Jogress Evolution:
Jogress is a fan made work which means Join and Progress. This is where two digimon at the mega level or beyond create a digimon that is beyond the mega level. For example, WarGreymon and MetalGarurumon Jogressed to create Omegamon/Omnimon, and Imperialdramon Fighter mode and Omnimon jogressed to create Imperialdramon Paladin mode. AND Gallantmon/Dukemon and Grani Jogressed to create Galllantmon Crimson mode, Shine Greymon and Masura's spirit Jogressed to create ShineGreymon Burst mode. I think you get the gist of it. The same rules that apply for DNA Digivolution apply for this too.

If you have any more questions concerning D-EXP or evolutions please PM or post in Questions/Suggestions and I'll edit this post.