In view of the recently reported nighttime (motorized) bicycle crash, here is a review of how to be safely lit when riding at night. Statistically, nighttime riding has a much higher crash rate than daytime riding, although the studies do not distinguish between those riders who were properly lit and those who were not.
The basic principle of being properly lit at night is to have at least one white/yellow light pointing forward, and one red light pointing back. Reflectors are not enough. Riding on lit streets is not enough. Beyond that, everything is details. Sadly, large numbers of riders are not aware of these principles.
For front lighting, there are two objectives. One is to be able to see in the dark, and the other is to enable others to see oneself. A light capable of actually enabling one to see in the dark sufficiently to ride of course also allows others to see oneself. The faster the planned riding speed, the more powerful the light needs to be. Front lights are typically mounted on handlebars or atop a rider's helmet.
In the recent past, battery powered flashlight-style incandescent bulbs were popular for front lights, with projector bulbs used for higher-end systems. The development of white LEDs (light-emitting diodes) has reduced their popularity, since LEDs put out similar amounts of light while consuming less power. One can typically get a feel for the amount of light the incandescent bulbs produced by looking at their power consumption; 1-2.5 W is good for being seen, and 6-15+ W to see. LEDs vary tremendously in terms of efficiency, therefore they are harder to compare. When published lumen figures are available, typically 100+ lumens is enough to see, and anything less is enough to be seen.
Rear lights are practically all red LEDs nowadays, and have been so for many years since they are so efficient. A single set of batteries easily lasts for hundreds of hours. The most important item when it comes to rear lights is to simply have one that works. Since a rear light can fail without the rider noticing, it may be advisable to have two or more for a measure of redundancy, especially for the commuter. The other item is to have them aimed correctly. Generally they should point straight back, since LEDs are highly directional and light that goes up to the sky or sideways isn't going to get people's attention. There is an ongoing debate over the merits and drawbacks of flashing versus steady rear lighting, and to a lesser extent, front lighting.
In the daytime, bright clothing is more visible than proper lighting, but certain lights are still visible during the day. Some riders choose to run lights during the day for this reason.