Visually, a verified account differs from a un-verified account by displaying a blue confirmation seal.
Blue confirmation seal
Right now verification feature is under Beta testing, this means twitter is verifying accounts of a small set of people; all well known in their respective fields. Twitter has till now verified a thousand odd accounts( belonging to celebrities and well known users).
This limited approach from twitter is due to the fact that verification needs resource allocation (like calling, checking address, tracking account etc). In addition, as impersonators normally impersonate well known people, hence twitter has chosen the easier way out.In future more accounts will be verified, though it should be believed that verification will always be done in special cases.
Share of Verified Accounts in all twitter accounts
As said, a very tiny or miniscule number of twitter accounts have been verified by twitter.
Out of the total Twitter accounts (estimated at 60 million) only 1,235 accounts are verified-- All belonging to celebrities and well known people. Thus among all twitter users 0.002% of Twitters users* enjoy the “verified” status.
Click here to see the Entire list of Verified Twitter Accounts. --------