is the use, without adequate acknowledgment, of the intellectual work of another person.
There is hardly a journalist alive who can place his/her hand on heart and say that he/she hasn't at some time been a plagiarist - it's almost impossible to write for a living and not do it. The problem is that it isn't always a clear-cut thing. I may have an original thought, or what I believe to be an original thought, when in fact I'm writing something that I've read a while back......only I've forgotten that I read it if you see what I mean. The internet is certainly a factor here - most writers (myself included) use the internet for research, and it's very tempting to snitch the odd sentence here, or the occasional phrase there. In the main it's harmless, and in some cases it can be positively flattering if you're the one being plagiarised.
The danger area is when you're writing a factual article. Facts are in the public domain, once established a fact belongs to everyone, but the way you write about facts - the ebb and flow of your prose - is yours and yours alone. If someone else uses the same ebb and flow, or even a part of it when writing about the same fact then yes, plagiarism is the result.
There comes a point of course when plagiarism ceases and good old-fashioned copyright theft begins, and that's become much more common with the advent of the internet and the copy and paste facility. It's too easy to grab entire chunks of someone else's work and plonk it straight into your own gem of creative writing. When that happens there's a copyright breach, and apart from being a cheat you're technically liable to be sued if you make money from someone else's original work.