In case of Rebuilding, it drop the particular index and again recreate it.It removes fragmentation, reclaims the disk space by compacting the pages based on the specified or existing fill factor setting, and again reorders the index rows in those contiguous pages. We can rebuild all the indexes of the table within a single transaction by specifying the ALL with it.
ALTER INDEX ALL ON [tablename] REBUILD
In case of Reorganization, it defragments the leaf level nodes of indexes by physically reordering the leaf-level pages to match the logical, left to right, order of the leaf nodes. It uses minimal system resources and also compact the index pages
ALTER INDEX [Indexname] ON [tablename] REORGANIZE
ALTER INDEX ALL ON [tablename] REORGANIZE
Rebuild should be used when the fragmentation index is greater than 30% and reorganization option should be used when the fragmentation index is between 5% and 30%. Rebuilding of an Index can be done online or offline. But to achieve the availability of the index, rebuilding should be done online. Reorganization can be done online.