I am new to component building, and I have noticed that other components have a comment on top of their units like agreement, terms and condition etc. whether its free or commercial.
How important is to register(license) your own component and how/where? and when do you need to register it?
How about freeware? Must have a license too?
how do we know that they're using it?
+1 vote to all good answers
I think it's a must to always have a license for anything that you distribute publicly. Even if it is freeware take up at least a license that ensures that you are never responsible for damages. There is a wide choice of suitable licenses you could use such as MPL/GPL and so on.
The license is an agreement between you and your customers. It is not important if the library is commercial with $1M price or freeware. The person who has started to use it, got it from you (copyright owner). So, he/she is your customer. And, as a customer, he/she may expect something from you. To make these expectations meet your own requirements / thoughts, you have to have an agreement with your customer.
To simplify things, take some existing license text as a base for your own one. If in there you find something missing which is important to you, then just put that into the text. For example, you may give the full right to change the copyright text and redistribute the library as the customer wants it. Or you may prohibit that.
The library units are a part of your library. To make that clear for others, you may put a comment at the unit top. There you may say, "this unit is a part of the library Xxx. For more details check the license text". Or something like that.
Even if you do not ask money for the use of your code, it is important to claim your copyright and to tell people what they can and cannot do when using your code. That is what a licensen does. Of course it doesn't guarantee that everybody will abide by your "license", but at least they can't claim ignorance or your failure to state what is allowed/acceptable. Some blog authors have phrases along these lines: "You can do use this code however you see fit, even without attribution, except you can't claim it as your own."
Edit: And as Remko points out a disclaimer to avoid damage claims can be pretty important too.
While its not a requirement for you to add this information to the top of any unit you release, it is good manners. It easily tells anyone who might be interested in using your unit, any constraints on such usage (such as not using in commercial products, or a notification be placed in the about of the main product... or even who to send a beer or postcard too).