To me, business casual equals casual slacks (think khakis or the equivalent), a button-down shirt, and loafers. Business casual is an intermediate step between jeans and what you'd wear to a formal job interview for a white collar job.
While, as others have indicated, you do not want to dress in something that you, as an individual, are not comfortable in, you also want to convey a professional image. In some circumstances, jeans can be considered business casual, but only in the right setting, worn by the right person, and combined with the proper shirt and shoes. In a business casual setting, a jeans/t-shirt/tennis shoes combination is a definite no.