Hey, since TO is on my mind just now, I suggest you take this question to TrainOrders, you are more likely to get responses.
But there are basics. A semaphore is a type of signal; signals are loosely speaking like traffic signals for cars, telling them whether to go or stop. IIRC, the red/yellow/green of highway signals came from the RRs who developed the concept first. A sempahore gives its indication by position of the arm instead of color or position of light (some semaphores are combined arm and light signals). A yard limit sign indicates the end of a yard and I believe it governs how far a train switching in a yard can go before it has officially exited the yard and thus is governed by other rules.
Obviously I am not sure of these things but I am not one to know the details of the nuts and bolts.
You should spend some more time on the web searching as there is a lot of detail available.