J-rock is 'japanese rock' but shortened. It is basically rock music from japan. J-rock has progressed over the years, and now some j-rock albums you are now able to get in the USA more easier than before.
Most j-rock bands are found out over the internet, or animes. For instance, the j-rock band FLOW does the opening song for the anime Eureka Seven, The Pillows did the ending song for the anime FLCL, and Asian Kung-Fu Generation did several opening songs for the anime FullMetal Alchemist, all making the bands more popular and well-known throughout the USA.
Some people think j-rock is in it's own genre, but it is NOT it's own seperate genre of music. J-rock is just used to tell you the music is from Japan.