Many people have different definition for identiy management.
Access management is a little easier to explain.
Access management is pretty much security. So, if Jim Smith is an administrator, and Jim Smith has rights to view the administration application, then Jim Smith will be able to view it. If Joan Ark does not have administrator rights, she will not be able to see the app. Access management is like security.
Identiy management is much different. I actually work on the industry leading identiy management tool, and my take may be different from others, but here goes.
Imagine William Gates bounces a check at your bank for $50. Do you send him a snotty email? Maybe William Gates has another account at your bank, that is under the slightly different name Bill Gates, and it has $10,000,000 in it. Now, do you want to send him a snotty letter?
An identity management systems will recognize where a person has perhaps opened up various accounts, or registered multiple times to a site or organization, with slightly different names/email address/passwords or addresses. A good identity management system will merge all of these different 'profiles' into one, single, IDENTITY.
Identity management can be used in many ways, from "know your customer" type applications that can monitor purchasing patterns, or for internal systems that can do password and resource management. In my slice of the identity management pie, I try to find 'bad guys' such as terrorists, fraudsters, con artists and criminals who are trying to obfuscate their identity. This is a non-obvious type of identity resolution, or entity analytics, that can be very useful for minimizing fraud, and identifying terrorists. If anyone is more interested in it, please message me through the JavaRanch.