If someone knows the password to your phone, can they access information on your phone (e.g. contacts, text messaging etc.)?
It depends. Are you talking screen lock password or Google password? A screen lock password is only useful to get into the phone to which the screen lock password is attached. So you would have to physically possess that phone. If someone has your Google password they have the potential to get whatever is saved to your Google account, as well as setup a phone with your account. This is why you want to enable 2-step verification and a strong password for your Google account.
Thanks for the quick reply. Just what I wanted to know.