There's a simple explination of fields on the MSDN site here.
This topic describes fields, which are objects or values contained in a class or struct. Fields allow classes and structures to encapsulate data.
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Fields store the data a class needs to fulfill its design. For example, a class representing a calendar date might have three integer fields: one for the month, one for the day, and one for the year. Fields are declared within the class block by specifying the access level of the field, followed by the type of the field, followed by the name of the field.