value type and reference type

http://visualbasic.about.com/od/usingvbnet/a/nullabletypes.htm

Value Types - Stored Directly on the Stack

The first way to store information is to save the actual value itself in the stack. Types that are stored this way are called value types and include most of the fundamental types you use. Here's the complete list of nullable value types:

• Byte
• SByte
• Short
• Integer
• Long
• UShort
• UInteger
• ULong
• Single
• Double
• Boolean
• Char
• Date