Is there any significance to shaving the head? I have noticed when I look at Buddhists, or Buddhist monks, that they have shaved heads. Is there any significance to that?  If so, then is it an obligation for Buddhists, or at least Buddhist monks to shave their heads?
First of all, lay Buddhists are not required to shave their heads, only the monks and nuns.
In most Buddhist traditions it is a custom/rule that when you become a monk or nun (a.k.a. Bhikkhu) you have to shave your head.
There are also monastic rules that say that a Bhikkhu is not allowed to grow hair beyond a certain length or time.

The hair of the head should not be worn long. It should be shaved at least every two months or when the hair has grown to a length of two fingerbreadths — whichever occurs first, says the Commentary.
The beard should not be grown long, although — unlike the hair of the head — there is no explicit maximum length, unless the two month/two fingerbreadth rule is meant to apply here as well.

(source: page 14 and 15 of the Buddhist Monastic Code II)
Several reasons have been given as to why these rules exist:

One of the first things Gautama Siddhartha (who became the Buddha) supposedly did when he left his palace and started looking for a way to defeat old age, sickness and death, was to shave off his hair and beard. Bhikkus show their commitment by doing the same.



Shaving our head symbolizes cutting off confusion, hostility, and attachment (source)


Shaving your hair removes the risk of vanity and allows you to focus on more important things than combing and fixing your hair every day.

Hygienic reasons

By looking less attractive, celibacy (another monastic rule) becomes easier (at least that's what this author claims, I'm not sure about the validity of this myself).