Examples of conjunctive adverbs: "also", "therefore", "however", "nevertheless", "meanwhile".
Unlike subordinating conjunctions, they do no mark the presence of a subordinate clause. Moreover, they are entirely optional, in that they can be generally omitted without affecting the whole sentence grammatically.
An example:
Some students like grammar. HOWEVER, others think it is boring.
Some students like grammar. Others think it is boring.
Unlike subordinating conjunctions, conjunctive adverbs are more mobile:
The experiment was disastrous. Nevertheless, we resolved to try harder.
The experiment was disastrous. We nevertheless resolved to try harder.
The experiment was disastrous. We resolved to try harder, nevertheless.
