Gmail Labs “Search Autocomplete” feature is designed to make searching in Gmail much easier. But if you’re a keyboard junkie, here’re some search operator for you: "in:important" — get all items labeled "important";  "is:unread in:inbox" — all unread messages; "to:me is:starred" — all starred messages sent directly to you; "is:chat from:heather" — all chat conversations you had with Heather; "is:starred -in:inbox" — shows all your starred messages that aren't in your inbox (a good way to find anything important that you might have accidentally archived); "from:elliot filename:pdf" — shows all messages from Elliot that have a pdf attachment

Source:→ Gmail Blog