Archiving is a process when you archive old master/transactional records which are not in use any more from long time. You archive those records keep on some backup device. These records can be displayed, and only Display, can never be reloaded in tables again physically. 
This will free up the space in tables from those records which are not in use any more but exist in tables physically.

Reorganization basically removes deleted data from tables. During different transactions you delete some records, which are deleted virtually but not physically. Even Archive process delete those records virtually.
So you run Reorganization to free up the space consumed by deleted records. 

This make the difference between the two clear.

Now about reorganization.

It depends on which OS and DB you are using. 

If Oracle, SAPDBA is best tool to reorganise.

In addition to above, you can reorganise the whole database, but it takes time depending upon your database.  Some times it can be more than a day or you can reorganise a table suggested in Early Watch system.

It all depends on you.

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