What is SPAM in simple words?

The word "Spam" as applied to Email means Unsolicited Bulk Email ("UBE"). Unsolicited means that the Recipient has not granted verifiable permission for the message to be sent. Bulk means that the message is sent as part of a larger collection of messages, all having substantively identical content. A message is Spam only if it is both Unsolicited and Bulk.

unwanted email / messages (mostly advertisements) sent to you

Unsolicited or undesired bulk electronic messages, such as e-mail spam. In more words: Spamming is a kind of network abuse. It's the abuse of any electronic communications medium to send unsolicited messages to someone in bulk. While its definition usually extends to any unsolicited bulk electronic communication, some exclude from the definition of the term "spam" messages considered by the receiver (or even just the sender) to be targeted, non-commercial, or wanted. In the popular eye, the most common form of spam is that delivered in e-mail as a form of commercial advertising. However, over the short history of electronic media, people have spammed for many purposes other than the commercial, and in many media other than e-mail. Spammers have developed a variety of spamming techniques, which vary by media: e-mail spam, instant messaging spam, Usenet newsgroup spam, Web search engines spam, weblogs spam, and mobile phone messaging spam.

pork

Unsolicited or undesired bulk electronic messages, such as e-mail spam. In more words: Spamming is a kind of network abuse. It's the abuse of any electronic communications medium to send unsolicited messages to someone in bulk. While its definition usually extends to any unsolicited bulk electronic communication, some exclude from the definition of the term "spam" messages considered by the receiver (or even just the sender) to be targeted, non-commercial, or wanted. In the popular eye, the most common form of spam is that delivered in e-mail as a form of commercial advertising. However, over the short history of electronic media, people have spammed for many purposes other than the commercial, and in many media other than e-mail. Spammers have developed a variety of spamming techniques, which vary by media: e-mail spam, instant messaging spam, Usenet newsgroup spam, Web search engines spam, weblogs spam, and mobile phone messaging spam.

unwanted email / messages (mostly advertisements) sent to you

pork