If I install geth, will it download the entire blockchain?
It depends on the synchronization mode you choose. From this answer:
"Full" Sync: Gets the block headers, the block bodies, and validate every element from genesis block.
Fast Sync: Gets the block headers, the block bodies, it performs no validation until current block - 1024. Then it gets a snap shot state, and goes like a full synchronisation.
Light Sync: Gets only the current state. To verify elements, needs to ask to full (archive) nodes for the corresponding tree leave.
No, installing geth does only install the binary go-ethereum client.
The blockchain only gets downloaded if you run geth. By default, the full blockchain gets downloaded if you just run
$ geth without parameters. So, it's a 'yes' if that was your question.
You can control the behavior of the synchronization by adding the
--fast or the
--light flag as explained by Herman Junge.