If you enable saving your chat history in Gmail, your conversations from Google Talk are saved as standard emails. To make them stand out from the rest of messages, Gmail adds two labels: chat and chats.
If you want to search your chats, you just have to add label:chat to your query. If you want to search only your mail, exclude the chats by adding -label:chat.
Gmail has many other hidden labels that can make your life easier, so you don't have to use the advanced search:
* label:inbox - search only your Inbox. It's a good idea to keep your Inbox clean.
* label:star (label:starred) - search only your starred messages. You can use the star button like a bookmarking system. Star only important messages.
* label:draft (or label:drafts) - search only unfinished messages. Drafts could be used for storing notes or emails that must be sent later.
* label:sent (or label:sentmail, label:sent-mail, label:sent mail ) - search only sent mail. It may be useful to restrict your search only to messages you've sent, or only to messages you've received if you add a minus.
* label:all (or label:allmail, label:all-mail, label:all mail ) - search all your mail, including messages from Trash (label:trash) and Spam (label:spam).
* label:read and label:unread. Restrict your search only to read mail or unread mail. For example, to find all your sent messages that have unread replies, you can search for label:unread label:sent.
It's nice that you can combine all these labels with user-defined labels in a query. To show results that have both label A and label B, just search for label:A label:B. To show results that have one of the labels A and B or both, just search for label:A OR label:B.
Update: Instead of label:A, you can just type l:A. [Thank you, Busse].