What does the --fast flag does? I read somewhere that it doesn't download transactions of the blocks rather just state downloads? But what are state downloads?
What does geth use cache for? It can simply put the downloaded data in filesystem right?
What is jitvm and jitvm cache? And also why we need to force it?
From the Homestead Guide:
--fast
This flag enables fast syncing through state downloads rather than downloading the full block data. This will also reduce the size of your blockchain dramatically. NOTE:--fastcan only be run if you are syncing your blockchain from scratch and only the first time you download the blockchain for security reasons. See this Reddit post for more information.
There's also a lot of detail on fast synchronization algorithm on github. Here's a quote:
Instead of processing the entire block-chain one link at a time, and replay all transactions that ever happened in history, fast syncing downloads the transaction receipts along the blocks, and pulls an entire recent state database.
--fast