Mmmm, cookies! Tasty little things, especially with chocolate chips. Oh, wait! Wrong kind of cookies!
There have been rumors going around for a long time (about 15 years now) that cookies are evil files that should not be on your computer because they are used by spammers or hackers to spy on you. Nothing could be further from the truth. In actuality, you could basically say that nothing is put into a cookie that you did not put there yourself. (I'll explain this in a moment.) Further, cookies are encoded so that only the website that created the cookie is able to decode it and see what it contains.
Now, to start, let's talk about what a cookie is. It is essentially a glorified sticky note for a website so that it remembers who you are, what you looked at the last time you were at that particular site, and whether you still have anything in your shopping cart. It is not an executable file such as a program (or a virus). It is not a piece of spyware that is trying to figure out your passwords or find some way to lure you into giving away your credit card numbers. It is a harmless file.
A perfect example is when you go to Amazon.com and look at, say, books regarding the Revolutionary War. The website takes note of the last three books (maybe more but, for argument's sake, we'll say three) and who you logged in as so that the next time you log in, it can suggest some new books on the same topic that might interest you or it might also bring up some books regarding the Civil War.
To extend this further, let's say that while you are at Amazon, you decide to order a couple of the books. The website has too much going on for it to remember who is looking at what and who is ordering which items, so it sloughs off that responsibility to your computer. It writes the sticky note mentioned above to keep track of what you've looked at and another sticky note to keep track of what you have added to your shopping cart. The next time you add something to your cart, it just appends that item to what is already there. When you check out, it reads back everything from your cart's cookie and makes sure that everything is in stock.
So, why have people over the years gone berserk over cookies? Because, just like any new technology that they do not understand, they think that it is something evil or harmful. Remember when people thought that cellular phones were going to cause cancer or would somehow cook our brains? (The transmission power from a cellular phone is measured in milliwatts; it takes a microwave oven 1.5 kilowatts to boil a cup of water, 100 thousand to millions of times the amount of power from a cellular phone's transmission!) It's the same issue with cookies.
There are still those out there who have programs that go out to remove their cookies every time they shut their computers down or set their web browsers to clear their cookies (and other historical information) from their computers each time they exit the browser. It does no harm but it really does no good either and is inconvenient as well. I had a client recently complain that she could no longer use the Internet banking website like she was able to do just a few weeks ago. It turns out that before I went out there, she had someone else come out who was a cookie-hater and, thus, told her that she needed to have her Internet Explorer set up to delete all cookies each time she shut down Internet Explorer and, on top of that, she needed to have the browser set up so that no cookies would be accepted. Well, online banking requires that cookies be set as part of its security authentication routine, so when she tried to log in with the settings that she had, the bank wouldn't let her in. Changing the settings to what they should be (Accept First-Party Cookies, which means to accept them from the website that you are currently visiting), the bank's website worked just fine and she could do what she needed.
The best advice when there is a big brouhaha about computers or the Internet or some sort of technology is to talk with someone in the industry who actually has a good idea of how the specific file/technology functions and get his opinion about whether it is harmful or not. Oftentimes, you will find that the monster is about as real as the abominable snowman.