More than simple seniority, senpai implies a relationship with reciprocal obligations, somewhat similar to a mentoring relationship. A kōhai is expected to respect and obey their senpai, and the senpai in turn must guide, protect and teach their kōhai as best they can. Senpai/kōhai relationships generally last for as long as the two people concerned stay in contact, even if the original context in which the senpai was senior is no longer relevant.
Romantic crushes on senpai are fairly common in Japan, and this is reflected as a popular 'fetish' in shōjo and romantic stories.
In translated English anime and manga, senpai/kōhai are more commonly used in fansubs and scanlations, as official translations tend to use familiar English terms.
The word is rendered as sempai in the original version of Hepburn, while it is rendered as senpai in revised Hepburn and modified Hepburn. Pronouncing it "sempai" is more accurate to actual Japanese.