A good password is consisting of minimum 8 random characters, it should includes capital, small letter, number and special characters. For added security you should have one password for one login to a website/account especially for important logins such as internet banking, email account, hosting login, etc.
Now with so many accounts we have nowadays, say internet banking, paypal, ebay, facebook, etc. it would be difficult to have one password for each account with 8 random characters at minimum. That is why we need a password manager.