Hello Ranjeet,
It's probably not a good idea to think of all words ending with -ly to be similar. In general however, these words are adverbs. Adverbs are words that modify (or describe) actions (verbs), adjectives or other adverbs. In fact, adverbs (like all adverbial modifiers) describe when, where, how, under which conditions, or to what extent . Below are a few examples
I: John supposedly did the work. Here, the adverb 'supposedly' tells you how he did the work. He didn't do it 'certainly' but supposedly. This adverb modifies an action (verb to do)
II: John's work is greatly appreciated. Here the adverb 'greatly' tells you the extent to which the work is appreciated. The work is not 'barely' or 'somewhat' appreciated, but 'greatly' appreciated. This adverb modifies an adjective (appreciated)
III: John's business almost completely failed. Here we have two adverbs. 'completely' tells you the extent to which the business failed. It didn't fail 'partly' but 'completely'. This adverb modifies an action (verb to fail). The 2nd adverb is 'almost', which tells us how 'completely' the business fail. It didn't actually fail completely, but 'almost' completely. The adverb 'almost' modifies another adverb (completely).
I hope this helps,
-Patrick
