-----o----o-----o-----o-----o----- Series Circuit
------------------------------------
¦ ¦ ¦ ¦ ¦
o o o o o Parallel Circuit
___¦____¦___¦_____¦____¦____
Your light bulbs are connected in Parallel so that if one of them goes out it does not affect the rest.
Older Xmas Tree lights were often connected in Series so that if 1 failed it took bloody ages to find the faulty one and get the lights working again.
Above diagrams are not very good but you should understand what I mean !!
Regards to you and the Builder !