The Trust levels are related to policy files using the Security Policy configuration element and are valid at three levels namely the machine level, application level, and in an application Web.config file. The Trust levels can be included or removed by adding entries to the configuration section that specifies the trust level name in Aspnet_isapi.dll file.
There are five trust levels known as Full, High, Medium, Low and Minimal Trust Level. Full trust allows your code to do anything it likes within the framework, meaning that all .NET permissions are granted. There are many web applications, such as ASPDotNetStorefront, which require full trust in order to function properly.
The main difference between each trust levels is as below.
Full trust - Allows your code to do anything it requires as all permission are granted.
High trust - same as ‘Full trust’ except that your code cannot call into unmanaged code, such as Win32 APIs and COM interop.
Medium trust - same as ‘High trust’ with the exception that your code cannot check any part of the file system except its own application directory.
Low trust - same as ‘Medium trust’ except that the code cannot make any out-of-process calls, such as calls to a database, network, etc.
Minimal trust - the code is restricted from anything but the most trivial processing like calculating algorithms.