Microphone signals: The lowest signal level typically found in a studio. Usually an XLR-type connector is used. These are very low level signals that are usually balanced. Microphone level signals should only be connected to microphone inputs. Microphone inputs will usually overload if a hotter signal such as a line level signal is connected. A microphone signal is usually too weak to drive any line input (either -10 or +4).
-10dB line level signals: Usually either a 1/4 inch or RCA type connector is used. This is the most common signal level used to make connections in home studios and they are usually unbalanced. Many signal processors, tape decks, and mixers use -10 level signals. Signals with this level can sometimes be connected to a microphone input but this should only be done when you are desperate and there is no alternative. They can also be plugged into +4 level inputs but again, this is not ideal. Microphons cannot be plugged into -10 inputs. +4 signals can be connected to -10 inputs, but it is not recommended and they will often overload the -10 input. This is in addition to the impedance mismatches and other compromises that you make when connecting dissimilar signals. Just because you can do it doesn't mean it's a good idea. For best results -10 level signals should only be connected only to -10 inputs.
+4 line level signals: These are usually found on either XLR or TRS connectors and are usually balanced. Many high end processors, mixers, and tape decks are capable of interfacing at these levels. Because of the widespread use of -10 equipment, these +4 devices are often capable of switching their levels to -10 for compatibility. While a +4 signal can be connected to a microphone input, normally this will produce severe distortion (as in the case above). A +4 signal can sometimes be connected to a -10 signal with usable results but it is a compromise. A microphone level signal is not usable when connected to a +4 input, so don't even try it. A -10 signal can sometimes be connected to a +4 input but the results will vary depending on the range of the input level on the receiving device.
If you gain an understanding of these three common signal levels and where and when they can be used, you will have much better experiences interfacing your equipment in the future. Please do not mistake the facts given here as all of the facts. This is a very deep and complex subject and there is a lot to absorb if you are to really understand it. For practical purposes however, the information above should help you get your equipment up and running.