Bike lights fall into two definite categories; lights to see with, or lights to be seen with.
If you are commuting in a well lit area, you need safety and visibility, and there are many lights available which feature flashing modes to draw the attention of other road users. Lights such as these are not just for dark nights; they are also useful in winter, and on summer evenings. They offer long battery life, are lightweight and very affordable.
If you need to see with your light, more lumens are required (preferably over 600), and a focused beam which allows you to spot hazards early on. Given their high power output, they are generally USB-rechargeable, and can be dimmed to preserve battery life. A quality rear light is also important, to be seen by others behind.
Mountain Biking at night offers an exciting challenge, with lights which allow you see see far and wide. With little ambient light, but many hazards and high speeds, exceptional lights are a necessity. However, this means that there is generally a separate battery, and a greater cost.