Qt signals (QueuedConnection and DirectConnection)

You won't see much of a difference unless you're working with objects having different thread affinities. Let's say you have QObjects A and B and they're both attached to different threads. A has a signal called somethingChanged() and B has a slot called handleChange().

If you use a direct connection

connect( A, SIGNAL(somethingChanged()), B, SLOT(handleChange()), Qt::DirectConnection );

the method handleChange() will actually run in the A's thread. Basically, it's as if emitting the signal calls the slot method "directly". If B::handleChange() isn't thread-safe, this can cause some (difficult to locate) bugs. At the very least, you're missing out on the benefits of the extra thread.

If you change the connection method to Qt::QueuedConnection (or, in this case, let Qt decide which method to use), things get more interesting. Assuming B's thread is running an event loop, emitting the signal will post an event to B's event loop. The event loop queues the event, and eventually invokes the slot method whenever control returns to it (it being the event loop). This makes it pretty easy to deal with communication between/among threads in Qt (again, assuming your threads are running their own local event loops). You don't have to worry about locks, etc. because the event loop serializes the slot invocations.

Note: If you don't know how to change a QObject's thread affinity, look into QObject::moveToThread. That should get you started.

Edit

I should clarify my opening sentence. It does make a difference if you specify a queued connection - even for two objects on the same thread. The event is still posted to the thread's event loop. So, the method call is still asynchronous, meaning it can be delayed in unpredictable ways (depending on any other events the loop may need to process). However, if you don't specify a connection method, the direct method is automatically used for connections between objects on the same thread (at least it is in Qt 4.8).

in addition to Jacob Robbins answer:

the statement "You won't see much of a difference unless you're working with objects having different thread affinities" is wrong;

emitting a signal to a direct connection within the same thread will execute the slot immediately, just like a simple function call.

emitting a signal to a queued connection within the same thread will enqueue the call into the threads event loop, thus the execution will always happen delayed.

QObject based class has a queued connection to itself