A senpai and kohai are generally similar in a lot of ways. They're usually fairly close in age, and have a similar occupation and social standing.The differences is when it comes to seniority and/or experience.
Two people may both be students at the same school, but if one is a first-year student and the other is a third-year, then that would constitute a senpai/kohai relationship.
Kohai in Japanese script: 後輩
