Google currently offers three different account types for their AdWords Advertisers. These accounts range from letting you mange a single AdWords account to managing a thousand different accounts with one username and password. These accounts all have their benefits and disadvantages.

Google’s AdWords Account

This is the most common and most basic Google AdWords Account. It is the first account you sign up with and gives you access to at most 25 different campaigns each containing up to 100 different AdGroups. This Account is ideal for people who manage advertisements for few webpages and allows a maximum of 50,000 to 60,000 keywords.

Benefits:

Great for advertisers who have few advertisements.
Easy user interface and easy to learn.
Low cost $5 Sign up fee.

Google’s My Client Center Account

I recommend watching this video by Google about the My Client Center account:

Google Video - My Client Center

Google offers AdWords Advertisers a different type of account setup called the My Client Center Account. This account allows advertisers to manage up to 1,000 different AdWords Accounts without having to sign out and sign in to each individual account to make changes. This account is perfect for those whose advertisements span over two or more different Google AdWords accounts and allows the account manager certain benefits.

Benefits:

Great for advertisers who have more than one AdWords account, because it allows you to view two different accounts simultaneously without having to log in and log out.
One login name and password can access up to 1,000 different Google Accounts.
Can run reports over several different Google Accounts rather than just one account.
Low cost $5 sign up fee.

Disadvantages:

May be a little confusing for novice advertisers.
Designed for advertisers who are interested in becoming a Google AdWords Professional.

Google’s Advertising Professionals Account

A Google Advertising Professionals Account is the most advanced account you can receive. Even though it is set up the same as a My Client Center account, it gives its advertisers more privileges than a My Client Center account. Advertising Professionals receive $6,000.00 in advertising vouchers from Google (60 vouchers worth $100 each). Advertising professionals are the only advertisers which can legally gain access to the Google Qualified Professionals logo, which can be used to advertise their professional status. Google requires advertisers to manage a My Client Center Account for 90 days, spend at least $1,000 in advertising during that period, and pass the Google Qualified Professionals Exam, before they will allow you to become a Qualified Google Advertising Professional.

Benefits:

Great for advertisers who manage several different AdWords accounts. Allows you to view two different accounts simultaneously without having to log in and log out.
One login name and password can access up to 1,000 different Google Accounts.
Can run reports over several different Google Accounts rather than just one account.
Google gives account holders $6,000.00 in advertising vouchers (60 vouchers worth $100 each). Can be used on any Google AdWords account less than two weeks old.
Receive access to the Google Advertising Professionals Logo to display on your website, business card, etc. to help bring in more clients.

Disadvantages:

Requires you to pass a Google Advertising exam, which is over 100 questions about all types of Google Paid Advertising. Exam costs $50 USD and you must make 75% or higher to pass.
Must retake the exam every two years to keep your Professional Account and Status active.
Less than 800 people world wide have become a Google Advertising Professional.