"Блин" is an euphemism for semiobscene word "блядь" (literally means different -- slut, whore, but being used separately can be translated as "shit!", "damn it!"); it is used when you want to swear but don't want to pronounce or to be heard to say that swear word-expression "блядь!", as it sounds alike yes "блин" (pancake) as being a neutral word is widely used -- in TV/radio, by kids in the presence of their parents and so on -- but still it is impolite to use it Being used in a sentence it adds a sense of disappointment, regret, irritation, annoyance, reluctance etc (For example: Зайдёшь в магазин за продуктами? -- Ой, блин, я кошелёк дома забыл. Как сходил в кино? -- Да, блин, я вообще туда не попал, билетов не было. Как прошла встреча? -- Ой, блин, не хочу рассказывать.) It's like to swear with "fork" instead of "fuck".

The word "Блин" may also refer to expressing your emotions and is equal to English "damn". Example: "Блин, я ошибся!" "Damn, I made a mistake!"

The Russian language has an expression: Первый блин всегда комом :) It means when you are cooking pancakes, the first one usually doesn't seem to be a pancake. :) That's why it may express the annoyance and bad fortune. Of how and when to use it, people have already answered to you.

the latter habit is considered (obviously) vulgar:) And actually the most educated swearing people around me often prefer (given the context is appropriate and everybody around has no objections about swearing words:))) the original swearing to this euphemistic. SOmething silly about this pancake, you know. And I forget, sometimes these words can be rather positive: 3) во б..! во блин! - more or less the same as "ого!".

A krasovdm said. Both words - the original swearing word and the euphemistic are used 1) as a standalone sweraing: "бл...!" - like when you accidentally hurt yourself or have just mad an unpleasant discovery 2) filler words, just inserted to certain places in your phrase... either to carry some emotion (mosly negative) or just because you, б... are, блин, used, блин, to speak this way, f*** it.