Do you mean that when you boot Ubuntu, you go directly to the desktop instead of a login screen?
If that is the case, then you need to enable the login screen.
(1) Enter System Settings into the Dash search field (or click on the System Setting sicon in your Unity launcher)
(2) Click on User Accounts (should be in the bottom right corner)
(3) In the top right corner of the dialog there will be a padlock icon with the word unlock or lock written beside it.
If it says Lock then it is fine.
If it says Unlock then click on it. You will be asked to enter your password.
(4) Click on your user account (there is probably only one – yours).
(5) Locate the field in the righthand side pane that says Automatic Login. Slide the bar to off.
(6) Restart your system and you should get the login screen.