Radioshack 555 Timers
The TLC is a newer version using a different type of semiconductor technology. The NE is a TTL chip while the TLC is a CMOS chip. It has different VIL/VIH/VOL/VOH and VDD levels, as well as IOL and IOH. As such, the TLC version cannot accept as high a voltage as the NE version, and it cannot drive as large a load directly. But the benefit is a much lower IQ, or standby current. There is some other differences in timing and other specs, but That's the main difference for hobbyists on general projects.

This can all be learned from the datasheets for each chip.