An essential part to the Japanese college experience is joining a school activity groups. Not surprisingly, university clubs are where young people form their closest social networks, some of which would extend into lifelong friendship. The intensity of relationships one can form in these activity groups makes it a great source of inspiration for many a Japanese teenage novel/ manga/ drama series.
University activity groups typically fall into three groups; competitive groups, and study groups, and circles:
Competitive groups are your college sports teams or competitive academic or musical teams, and are serious affairs. College sports in particular are prestigious in Japan, and some of the most talented college athletes do train seriously to forge themselves professional sporting careers. Social hierarchy within the groups are also typically strict, with strong senior-junior social rules guiding interactions within the team members.
The college sports scene in Japan have famous rivalries, such as the one between Waseda University and Keio University (the top private universities in Japan), and matches between these two universities are closely followed even by those not affiliated by to the schools.
Study groups (benkyoukais, or kenkyuukais) are more academic groups that are also fairly serious. Students come together to explore a topic in detail, be it traditionally academic topics like physics or mathematics, or more practical ones like robotics and finance, or more niche ones like specific anime or The Beatles. Some music groups also fall into this category, such as the more studious jazz groups.
Circles on the other hand are more casual student groups that get together mainly for social purpose. The types of circles run the gamut, from rock bands to tennis clubs to international dance groups to gaming societies, and social activities like day trips and drinking events are common bonding activities organized. They are non-competitive, though that does not imply students are less committed to their circle, or that the social hierarchy within the circle is definitely less strict than competitive clubs.
If you are planning to go to a Japanese University as a full time or exchange student, I highly recommend joining a student activity group while you are there. It would be a great way to meet fellow students (Japanese and non-Japanese) in a deeper and meaningful way.