What's the difference between septicemia and sepsis?

Same thing mostly: Septicemia as a term generally implies that there has been confirmed to be the presence of bacteria or other micobial agents present in the blood. Sepsis is generally a syndrome of severe vascular compromise. However, generally speaking the two terms are used interchangably.

Interchangeable: Sepsis is a systemic response to an infection. This infection could be anywhere. If the infection is in the blood and the blood culture grew an organism then it is called septicemia.

Same: Basically sepsis means the presence of an infection causing an immune response. Septicemia means the presence of bacteria in the blood stream.

Basically same: Sepsis is a term which means the body's reaction to an infection. There are specific criteria - like fever, high white cells, breathing fast, and a fast heart rate - that, when combined with infection, mean sepsis. Septicemia is an older term for the same thing.