As Ofir already wrote – if you try to read data from an object that some other thread is modyfying – you could get data in some inconsistent state.
But – if you are sure the object is not being modified, you can of course read it from multiple threads. In general, the question you are asking is more or less the Readers-writers problem – see http://en.wikipedia.org/wiki/Readers-writers_problem
Lastly – a critical section is an abstract term and can be implemented using a mutex or a monitor. The syntax sugar for a critical section in java or C# (synchronized, lock) use a monitor under the covers.