Passwords, especially complex passwords, can be difficult to remember.  When creating online passwords, there is often times specific criteria that is required to make your password acceptable. Typically requiring letter, numbers and special characters.  To fit this criteria, password generators are often utilized to produce 'secure' passwords such as 'PRAm8jl$iW'.  This password has 10 characters, letters, numbers and symbols, but can be very difficult to remember.  

This is why the practice of using a Passphrase can be very effective.  When you decide to use a Passphrase, you may choose a hobby such as traveling.  If you choose a phrase such as 'i like to travel' and then add in the required criteria 'I Like 2 Travel!', you have just gained 41 Million years of password security, with an easy to remember Passphrase.  Compare this to your 'secure' password 'PRAm8jl$iW' which only gains about 5 years worth of protection.  

No, when you use duplicate passwords for more than one account, if compromised, an attacker will have access to all of the accounts that use the same password.  

While a password is typically a short character set of mixed digits and letters, a passphrase offers the ability to have a long string of text with spaces.  The advantage of this is that a phrase is typically easier to remember than a password with random numbers and letters.