A system identifies a number of blocks, each of which handles specific tasks but together perform a specific function coherently. For example, an Audio Amplifier system could be broken down into seperate parts. You've got a Power Supply Block, Source Selection Block, Pre-Amplifier Block and Power Amplifier Block. These seperate blocks each perform a specific task but together they all work coherently to provide an "Audio Amp SYSTEM".
A signal can be considered as any information, whether analogue or digital which is the output of one device and the input of another.
These aren't official definitions they're just my own outlook on them. For definitions which are suitable for the course you're studying, you need to consult the course syllabus.
Brian