Explain the term lever or rocker or follower ?

                                                  

In Fig. AD (link 4 ) is a crank. The link BC (link 2) which makes a partial rotation or oscillates is known as lever or rocker or follower and the link CD (link 3) which connects the crank and lever is called connecting rod or coupler. The fixed link AB (link 1) is known as frame of the mechanism.

When the crank (link 4) is the driver, the mechanism is transforming rotary.