Over the last decade words with the root "пиар" ("пиар-компания", "пиар-технологии", "пиар-акция", "чёрный пиар", "пиарить") became quit popular on Russian media. In this regard, I would be interested to know differences in use of "пиар" and "пропаганда" or "реклама", and why words with the root "пиар" become so popular.
Пропаганда is about view-of-life, it's not about a person or a company. This could be about something like 'our country is the greatest' or 'to be healthy you need to run 5 miles every day' etc.
Реклама is advertising - posters on buildings or in the transport, banners in the internet etc. It is obvious that a person or a company paid for that.
Пиар also makes a person or a company more known - but in more subtle way. For instance a person may be known for organizing a music festival, or some special party each Valentine Day etc. So, it is not a straight-forward advertising, but a way to get people's attention.
Черный пиар could be some kind of activity to create a negative impression of a person or a company, e.g. there could be a series of articles or mentioning in media that 'they don't spend money on environmental protection' or 'they payed bribes' etc.
Пиар is a Russian rendering of PR (пи-ар) acronym, which stands for Public Relations. As far as I know it has a different meaning in Russian than Public Relations in English. Its a professional jargon among people that are a professionals in promotion and advertising. It become popular maybe because it contains the (new to Russian language) idea of organized activity of making company or a person more known to a wide audience. Before only реклама existed, but it does not cover all the required meanings.
Пиар has no negative meaning by itself, but when someone wants to say 'they are not so good as they look, they just want a positive image' they say 'это пиар', or 'они пиарятся на помощи бедным' (they help poor just to have positive image, not because they care about poor people) then пиар has negative meaning of course.
Usage of пиар can be traced back to 1996 - as you can see it became very popular during last several years. One of the first printed occurrences goes back to 1989:
Делал без причин, иногда даже во вред самому себе, своему, как бы сейчас сказали, «пиару» или «имиджу». [Генрих Боровик. [Послесловие к книге «Дело Кольцова»] (1989)]
I think it is one of the business-related words that become popular in post-USSR times like менеджер, проект, имидж, etc. that can be applied almost to any field of human activity (like art, architecture, technologies, literature and so on and so forth).
Actually, if we look on the examples of usage of пиар in 1990-ies, mostly they are connected with political parties. So, actually, at those times пиар (политический пиар) was a synonym for a "пропаганда" or "агитация" ("предвыборный пиар" = "предвыборная агитация", "пиар-команда" = "агитаторы"), but those are Soviet words, so usage of пиар allowed to sound modern.
However even in those times you can find examples like "после такого пиара клиенты стали обращаться в другой банк", "Деревня и райцентр не имеют пиара", "адвокатский пиар, с помощью которого будируется общественное мнение в пользу обвиняемых".
The differences of the word's meanings are (with the definitions):
пропаганда (in English "propaganda" has exactly the same meaning as it has in Russian):
- state or government lies (it seems like this meaning the word has gained in recent years, because I cannot remember any example of it in literature etc.)
- a systematic, organized complex of measures to spread ideas/ideology to the widest audience possible (you can encounter occurrences of "propaganda" with this meaning in literature, in historical (like министерство пропаганды в Германии / the Ministry of Propaganda in Germany, отдел пропаганды в комитете Компартии / the Department of propaganda in a committee of the Communist Party etc.) and other contexts (like "пропаганда здорового образа жизни / propaganda of the healthy life style", "анти-табачная пропаганда / anti-tobacco propaganda" etc.);
пиар (almost the same meaning as PR (Public Relations) in English, but often implies a negative attitude and usually is not used officially):
a complex of measures to promote something, gain popularity usually involving media. Or an instance of such a process. Can be used about a person, company, product. This has been borrowed from English (PR). Usually:
it is not used officially, that is you can hardly hear that a company has a department of "пиар", generally this is called something like "отдел по связям с общественностью / прессой и т.п." (literally: the department of liaison with the press, public etc., though in English, as I can remember this is commonly called something like "the Public Relations Department);
it does not involve sales, that is you don't (usually) say "пиар, чтобы продать товар";
it has a purpose to affect the general opinion of the audience about something, not necessarily intended to entice the audience to do something immediately. For instance: a company paid for a commercial on a TV channel, this is called "реклама", it is intended to prod the audience to buy the product. The company made a big investment somewhere, so that media covered this (or the company paid some journalist so they wrote something about it) this is "пиар".
реклама (in Enlish there are at least two words corresponding to this: advertisement, a commercial)
- a complex of measures to promote something or an instance of such promotion (like a TV commercial, newspaper advert, web site banner, advert in a bulletin board or on a light pole etc.). Usually this is about goods/services, businesses but can be of anything actually. Though it is very unlikely, that a government or state would be promoting their policy in such a way, which in its turn is done commonly via press-conferences, tv-shows on state-owned or affiliated media etc.
Regarding your second question about why words with "пиар" have gained popularity I can only guess. I think the reason is alike to one, which is behind the popularity of the words: логистика, менеджер, чернокожий etc. It can be that this words started to be used by people who use English a lot and then spread to other's vocabulary.