Audio output connectors 
Line level outputs can typically output signals up to 2V levels and ouput impedance of few hundred ohms. Speaker level outputs on soundcards have typically around 2W of output power and can drive 8 ohm sepakers or 32 ohm headphones nicely. The typical 3.5 mm output jack has the following pinout:
- How to solve ground loop hummign problems when connecting yout computer to your HiFi system
- Simple audio mixer for mixing signal from two soundcards
Audio input connectors 
Typical line level audio input connections are designed to accept audio signals in the range of 500 mV to 2V. Line level audio input connectors are high impedance inputs (typically around 10-47 kohm). The pinout is the same as in the line level output:
- CD-ROM audio connectors - how to connect CD-ROM audio to your soundcard
Microphones 
Most sound card inputs require a minimum signal level of at least 10 millivolts. Sound Blasters and some older 8-bit cards need 100 millivolts and supply bias voltage on their outputs to power the electret microphones (the only microphone type which works with them). This discrepancy means that if a typical professional microphone is connected to a sound card input, the user will have to shout into the microphone or hold it just an inch or so away (or both) in order to produce a strong enough signal for the sound card to "hear."
- Interfacing Microphones to Computer Sound Cards - application notes from Shure how to interface different microphones and why you can't use professional microphones with some soundcards
- How to connect a dynamic microphone to Sound Blaster soundcards - if you connect dynamic microphone directly to Sound Blaster microphone input you get unacceptably low signal levels but this simple circuit solves the problem
- Multimedia Microphones - how to use those microphones which came with soundcard
- Sound Blaster Microphone Preamplifier - how to connect dynamic microphones to Sound Blaster soundcards
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