Sorry for late reply..
This is detailed question but let me explain
Head term keywords are very high competitive such as Google Map, How to Blog, Blog CMS etc
But long tail is actually a strategy and in which we use long but low competitive keywords such as "Absolute Guide on SEO interview" OR it can be "Sony z5 Dual E6633/E6683 price and specification" it looks long even there searches would be lower but it is effective because there would be lower web pages on that same particular keyword. And you would be the one of them