You should read about "dynamic references" (also called "dynamic bindings"). A good starting point: http://www.salesforce.com/us/developer/docs/pages/Content/pages_dynamic_vf_sample_standard.htm
Basically if you have String fieldName = 'AccountNumber';, later in Visualforce you can refer to it directly:
<apex:outputField value="{!a.AccountNumber}"> or dynamically:
<apex:outputField value="{!a[fieldName]}">.
This is similar to having a.AccountNumber or a.get('AccountNumber') in Apex. Check out for example getting Value of a field by its Name in apex salesforce if you've never seen it.
You'll have to be careful around them because if your base object won't be account it will fail (for example there's no Contact.AccountNumber field). The example above deals with Accounts only but it'd be a good intro anyway.
Once you familiarize yourself with the basic idea you can explore world of fieldsets - predefined groups of fields that should come together - you can use them both for displaying and querying data, basically they're more powerful than hardcoded list of strings with the field names (like in first link) but still same idea.
At least you'll know now what keywords to look for ;)
