While bidding strategies have evolved over time, there are several core bidding types available in Google AdWords. Now with Flexible Bidding Strategies it may be a bit confusing as to what type of bidding is available and how it works. Cost-per-click (CPC) bidding has never been so advanced – and so complicated.
Manual CPC
This is the classic setting for having total control over bids with a focus on driving click traffic. AdWords will take the ad group default bid first, unless a different bid is manually specified at the keyword level.
Select "I'll manually set my bids for clicks".
Automatic
CPC
In this setting, advertisers focus on driving click traffic but
give AdWords control over individual CPC bids. Set a daily budget and AdWords
automatically adjusts your bids with the goal of getting the most clicks for
the budget. With automatic bidding, AdWords does all the work to get the most
clicks.
There is also the option to set a CPC bid limit. Setting a limit
can help control costs, but might also potentially limit clicks.
Select "AdWords will set my bids to help maximize clicks
within my target budget".
Enhanced
CPC
This bidding option is for the conversion-focused advertiser.
Conversion tracking must be enabled, so that based on conversion tracking data,
AdWords will automatically increase or decrease CPC bids to drive most
conversions.
Bids can be raised up to 30 percent for clicks that are more
likely to lead to conversions. Bids are lowered for clicks less likely to
convert.
CPA Bidding
With a focus on conversions at a specific cost-per-acquisition,
use CPA bidding. This is also known as Conversion Optimizer.
Advertisers who want to target a specific cost per
acquisition/conversion must have at least 15 conversions in 30 days to use
this. The conversions history allows AdWords to predict future conversions.
Because Conversion Optimizer automatically applies its own bid
adjustments, it isn't compatible with the new enhanced campaigns bid
adjustments across days, times, locations, and devices (except for mobile
opt-out at -100 percent.). Display bids also don't work. If you turn on
Conversion Optimizer with existing bid adjustments, they will simply be
ignored.
There are two advanced options for this bid type: Max CPA and
Target CPA. The Target CPA is the average CPA you are willing to pay, and Max
is the maximum per conversion. The Maximum CPA is scheduled to be discontinued
in 2014.
Google recommends a CPA, based on history, which can be used or
advertisers can set their own.
Source: http://searchenginewatch.com/article/2303776/Google-AdWords-Bidding-Strategies-The-Complete-Guide