The full title in English is Nana and Kaoru's SM Diary, by Ryuuta Amazume.
For those unfamilar with Amazume's works, he generally focuses on romantic relationships from a "mature" point of view - that being he doesn't beat around the bush. In Nana to Kaoru he descends from this model a bit, focusing on the awkward relationship between the two titular characters, Chigusa Nana and Sugimura Kaoru.
Despite being childhood friends, the two grew apart and radically different. Nana a model citizen and student, beautiful and well liked; Kaoru in a deviant - short, ugly, and irresponsible, with a rather obsessive interest in the art of bondage. The two come together again after Kaoru's mother hands Nana a bag of Kaoru's stuff, telling her to hold onto it for a while as a punishment for Kaoru. Nana peaks inside, and being curious, puts on a one-piece suit that seems to be a perfect fit for her...but the key to get out of it is nowhere to be found. She's forced to go to Kaoru for help, and this begins a new hobby for the two of them: Kaoru is able to practice his bondage techniques (tying ropes, mental manipulation, etc.), and Nana gets an outlet for her stress.
Despite being so heavy in S&M content, the series itself is surprisingly non-sexual, instead focusing on the maturing relationship and awkward tention between our hero and heroine, as Kaoru begins to grow up and mature and as Nana learns to relax and rely on others.
A spin-off series is also being published, also called Nana to Koaru, but published in Arashi magazine. This series follows Nana and Kaoru in the summer leading up to their senior year of highschool, as they travel together and temporarily live with some friends. While in the same vein as the original series, this one introduces the bondage aspect of the series as something sexual, rather than simply a bonding aspect between the two characters.
A one-shot anime OVA is currenly being produced, and a live-action television series is to begin airing in March.
Kaoru and Nana:
And the only other character worth mention in a summary like this is Tachi, a girl who runs with Kaoru later in the series.
It started in 2008, if I remember correctly, and flew under the radar for a while, but since it moved from a monthly series to a bi-weekly series its popularity has exploded, resulting in the aforementioned OVA and live-action adaptations.