I am something.
I can be seen, but sometimes cannot be read.
You can see a file in a folder or desktop or whatever, but you may not be able to read it (it may be empty, it may be encrypted, it may be a type of file for which you have no application, and so on).
I can convey love or hate
or joy or sadness
or any feeling.
Yes, any type of feeling might be conveyed by the contents of a file, be it an image, a song, a film, or just written text in a document.
Sometimes, I can help you.
Some files are applications or valuable data, that can be quite helpful to you.
But, sometimes, I can hurt you.
Some files contain malware, viruses, or the like.
But I can be stopped
if my home is not asleep.
The "home" is the computer, which if it's not powered off or in sleep mode, can be used to stop an application from running or a file from being used.
If you don't stop me,
I can be quite useful actually.
If you let a file be used, and it's a useful file, then it can be useful.
Depending on my family,
you can usually put me to some utility.
The "family" is the filetype - e.g. ".doc" / ".docx" for Word, ".zip" for archives, ".gif" / ".jpg" for graphics, and so on. Each type can have one or more applications associated with it which, when you try to open a file, the file is more or less literally put to that "utility" (application).
The building blocks of me,
sometimes they're millions or billions in number.
Files such as movies may be made up of millions or even billions of bytes.
Each of those blocks can be split further,
but not much further.
Bytes are comprised of bits, but you can't really divide things further than that.
My name can consist of one
or of two, if you want to be technical.
You might think of a file's name as the whole thing, or you might consider it as two parts - the base name plus the extension.
My family name is usually at the end of my name.
The extension is
usually always, really, at the end of the filename, if it is present; otherwise it's not an extension!
The (short) answer is a four-letter word.
Sometimes, my actual family can't be deduced from what you see at the end of my name. That is, it is disguised. Such instances may be harmful.
The extension, being part of the name, isn't always guaranteed to actually reflect the contents of the file itself. Also, many operating systems are, or can be, set up not to show file extensions, as it's usually redundant information already provided by the file's icon. But that opens the opportunity for someone to create an application that masquerades as an innocuous data file, tricking you into opening it to see what it contains and thus running the program instead. This is indeed quite often harmful.