In a few occasions if you use the review queues, you will find a post that is spam.
The definition of spam according to StackExchange (and the internet in general; source) is:
What makes something spam and when should I flag it? A post should be marked as spam ONLY when it contains an unsolicited advertisement.
It should NOT be marked as spam when:
The answer contains no useful information, such as an answer that says "I don't care about your problem". Flag an answer as 'not an answer' instead; if you find a weird non-question, then flag it 'for moderator attention' with a custom explanation.
It contains only gibberish, such as "fsdguejgkfdlk". Flag 'for moderator attention' with a custom explanation if it requires more detail. These flags are trivially handled by moderators and using spam/offensive for it poisons the data for anti-trolling and automated spam detection.
To give you a practical, recent example, that has been cleaned:
We are manufacturer of PyBOP (← original link removed) in China. If any needs, please do not hesitate to contact us. Thank you.
What shall I do when I see a post like this?
Editorial note: This post shall be included in our faq, so if you think something is missing, please edit it in, or if that is not possible, leave a comment below. It will be taken care of.