This lesson will cover the basics for connecting various types of equipment to your computer’s soundcard. But before continuing,
please read the following warning.
WARNING! Never plug powered outputs that are designed to power speakers (like those from a power amp or a powered mixer) into your soundcard. Doing so can cause serious damage at best and a fire at worst. I know of someone on another forum who plugged a 150watt amp into his soundcard and it caused a fire, melted the cables, and took out both his computer AND amp. So please be careful when dealing with this type of equipment!
Connections that are safe to plug into your soundcard are from non-powered mixer outputs, direct ouputs, aux outputs, effects inserts, headphone outputs, effects units, and even direct guitars/basses.
Connections that are NOT to be plugged into your soundcard should be labeled as "speaker outputs" or "cabinet outputs" (for guitar amp heads). Also if the connections give an "Ohm" rating (such as 4 Ohm, 8 Ohm, or 16 Ohm) these connections are also high power speaker outputs. Here are some examples of outputs that are NOT ok to plug into your soundcard:
If you are unsure of a particular connection, it is always better to be safe than sorry. So please ask before connecting it to your soundcard. 
The type of connectors mentioned in this lesson that you should be familiar with are...
1/8" Connector:
1/4" Connetor:
XLR Connector:
RCA Connector:
Some basic terms to be familiar with are defined below:
Mono and stereo:
The first diagram shows a mono output connected to a mono cable. Mono cables use two wires and can only carry one signal. An example of a mono cable is a guitar cable.
The second diagram shows a stereo output connected to a stereo cable. Stereo cables use three wires and can carry two separate signals. This can be used to carry the right and left stereo channels on one cable. An example of a stereo cable this is the cable on a pair of headphones, this cable is stereo and carries separate signals to the right and left speakers in the headphones.
The third diagram shows a stereo output plugged into a mono cable. In this case the right side is shorted to ground and only the left side goes through the mono cable. Likewise, if for example you were to plug a stereo set of headphones into a mono output, then you will only get sound in one speaker.
You can identify mono and stereo cables by the tip of the cable.
Stereo cables will have a tip, ring, and sleeve as shown above, whereas
mono cables will only have the tip and the sleeve with no middle ring section.
Male and Female connectors:
A
male connector has one (or more) exposed pin/terminal that plugs into a
female connector.
An example is this
1/4" to 1/8" adaptor, which has a 1/4" female connector on one side (left) and a 1/8" male connector on the other side (right).
Now that a few basics are covered, let's get started.