What is ECB(Edit control block) in Sharepoint?

While reviewing some document,i came across the word ECB.I am very new to the technology and i don't the terminology what they are talking about.I searched in google and after lot of research i came to know that it is control which appears when you drag the pointer where the documents are present in the document library for example.The drop down it comes is sharepoint terminology called as ECB (Edit control block).If you want,you can customize the control by adding your text. Below is the url you can refer for example: http://www.thespgeek.com/2011/01/how-to-add-custom-menu-item-in.html