While most of the music is in Japanese, many J-pop songs contain bits of English. Many J-pop song titles and artist names are also in English. Some songs are completely in English but still difficult to understand because of the artists' strong Asian accent (i.e. "SUMMER DAYS" by Do As Infinity). You don't need to know Japanese in order to enjoy J-pop.
Popular J-pop artists are: Ayumi Hamasaki, Utada Hikaru, Every Little Thing, Do As Infinity, BoA, day after tomorrow, and Puffy AmiYumi (known simply as "Puffy" in Japan).
The music is very addictive. In fact, it's all I listen to. With the exception of Puffy AmiYumi, J-pop cannot be found in American music stores. The best place to buy J-pop music from IMO is YesAsia.com. They have a wide selection and good prices.