If you don't, Wikipedia is always the best friend. But in short, bento is a packed lunch very popular in Japan. The main rule is that it must look cute and yummy. =P
Japanese moms prepare bentos for their husbands and their children and the students bring bentos at school for lunch. You can also buy bentos from shops, as fast snack! As result, bentos are omnipresent in japanese animes and mangas.
A typical $10 (1000 Yen) Bento, bought from a bento store in Tokyo Terminal (source: Wikipedia)
Lately the bentomania has spread also to the western Countries and it became quite popular in the USA, where now you can find shops that sell bento boxes and accessories. In Europe not so much... yet. Hopefully the trend will grow also here!
To make a bento it's good to have some useful tools, such as:
cups, I also use muffin silicon cups
dividers (food, e.g. salad, can also work as divider)
small containers and bottles for sauces and liquids
You don't HAVE to use these tools, but they make bento making much easier!
Internet is full of images of amazing artistic bentos that display anime characters and take 3 hours to prepare, they're not actually the real bentos... they're called charaben. And you don't have to make anything like this either! The main point is that bento is a balanced, tasty and nice looking meal.
There's no need to put asian food in your bento, you can put whatever you like (just remember that usually japanese bento boxes aren't suitable for microwave so fill them with food that can be eaten cold or at room temperature), even leftovers from the dinner. That's what I normally do :)
Another tip is to use veggies! They aren't just healthy, but they also bring colors and can be used as dividers e.g. baby carrots, bell peppers, cherry tomatoes.
The last advice I can give is not to stress too much over the bentos appearance. I love bento making because it relaxes me and I enjoy the process, that way I enjoy eating it later as well :)