The continuing etymology of the word "spam" is of interest to people with an interest in how words can develop, change meanings and take on additional, new ones. Originally used as a brand name for a tinned, processed meat, it was picked up and used in a Monty Python sketch and from there was adopted to refer to e-mail which was unsolicited, unwelcome and used to aggressively drive a message (usually commercial) regardless of audience. As time has passed, it has come to be used in terms of any unwelcome proliferation of information or advice. 

Spamming on Twitter does not only refer to commercial promotion, but still includes it. More than that, though, it has come to refer to anyone posting persistently with information that is either incorrect, irrelevant or just plain annoying. If you are accused - justifiably or otherwise - of spamming will inevitably result in you losing followers. For such reasons it is essential that you consider seriously what you are going to post before you post it. If you strongly feel that it is relevant to your followers, then tweet it. If you are not concerned by the prospect of losing followers, then posting things that may be considered irrelevant should not concern you. 

Bear in mind too that when it comes to relevance, telling people what they already know constitutes irrelevance. If you constantly posted that grass tends to be green, you would be spamming, and if you post well-worn political arguments in a feed mostly read by political experts, they may well accuse you of doing the same.