After dark, you need lights for two reasons: to see where you’re going, and to make you visible to other road users, especially motorised ones. You’ve a vast range of options, from tiny flashers that’ll help drivers spot you round town to high-power floods that light the way on dark lanes and paths.
Almost all lights now use light-emitting diodes (LEDs). LEDs are low-weight, efficient, and durable. A well-made light unit will last years. Most get their power from built-in rechargeable batteries. A few ultra-compact lights use replaceable button batteries that are best bought in bulk from eBay.
For maximum visibility, look for lights that put plenty of light to the sides so you can be seen as you emerge from a junction. For riding in truly dark conditions, you can't have too much light, but be ready to dim it so you don't dazzle oncoming road users.