I am not very much sure about how Magento Indexing works, But "Update Required" field is set to "Yes" immediately after i am done with re-indexing all indexes.

I have few queries regarding indexing
- On what condition status of an Indexer changes?
- Are various indexer available in Magento are dependent on each other?
- Why it shows "Update required = Yes" even it is in "Ready" status. ?
- Is indexing executed on daily basis?If yes, then from where we can change time of its execution?
- If a store receives couple of thousand request to update inventory via Soap API, do indexer are executed and updated for every request which is received to update inventory of a product?
Your valuable input are much appreciated.
Thank You.