Within the body of a method, all of these types of variables can appear. Many find naming conventions to be a helpful aid in getting a rapid understanding of a method's implementation. These conventions have only one purpose: to pour understanding into the brain of the reader as quickly as possible.
To understand a method, you need to understand its data. An important part of understanding data is understanding where it's defined - as a field, argument, or local variable. Naming conventions which distinguish these cases usually allow the reader to understand an implementation more quickly, since they no longer need to scan the class to determine where items are defined.
Some different styles are:
fBlah, _blah, blah_, this.blah, m_blah, myBlah
'C'convention is fundamentally different from the other conventions mentioned above. This convention is visible to the caller, while the other naming conventions, being confined to the implementation, are not.
The example code on this site usually follows these conventions:
BlahBlah(same as class)
See also Oracle's remarks
on naming conventions in general.