Can someone explain exact difference between trail,module and unit ?

Hi Santosh, you can think of it as follows:

• Trail = Lesson Plan with several lessons
• Module = A specific lesson broken out into several exercises
• Unit = An Individual exercise inside a lesson

Trails are great way to learn high level themes, such as Certification Topics, How to Use Apex, How to Build Lightning Components. 

A trail consists of several modules, where each module represents a specific lessons.  The modules are broken out into several units, usually 4-5, so you don't complete the module until you complete all units.   Each unit has a quiz or hands-on exercise that you can complete, and then once you complete the module you get a badge. 

Trails have progress, but they don't necessarily award anything for completing them. They are more of a container/lesson plan to list a grouping of modules that are helpful to go through together.