Can someone tell me more of this institution ?

And also what was its difference with the blades?

The original Dragon guard were from Akaviri and they served Reman Cyrodiil. Blades are what it became when the Dragon guard were gone and humans took their places. Basically it could be considered as a one and same order but it's human members are called "Blades" when Akavirian members are called "Dragon guard". (It's not mentioned which Akavirian race they were but I believe they may have been Tsaesci.)

I think most probably where Akaviri ( akavir humans , so asiatic , since its written that they also breen with local population , and it woudl seem strange to have imperials descendinf from an argonian like creature )

Anyway other information on what where they doing , duties, powers , organization , if they rode dragons etc?

I do not know much about their duties and organization. Only that they built the Sky Haven and Cloud Ruler temples and the Alduin's Wall. They served the Emperor and hunted dragons. But there's also small hints given to us in other info about Akavirian influence on Tamriel. The organization and tactics of Imperial legion are based on what they learn from Akaviri and the Fighter's Guild was based on idea founded by Akaviri Dinieras-Ves. These things suggests Dragonguard most likely was really well organized order with strict code of honor. I do not believe them raiding dragons - they were dragon slayers not dragon riders.