Old School - Analog
Before digital there was analog. Analog mixers are electronic components designed to accept various sources of audio which then give the user control over how loud the audio will be from a microphone, electric guitar, drum set, etc. The control is usually determined by what are called potentiometers or "pots". Pots used to look circular but later evolved into slide controls.
Complicated analog mixers are still often used in concert venues by sound engineers and even radio stations still rely on a "console" in on-air studios to mix the microphones and other audio sources.
Mixers are like Spigots for Audio
I mentioned mixers are important for sound location, too. If you imagine a Podcast (or radio show) as one continuous stream or line of audio, then you'd agree a show open, commercials, and a show close would come at different points during that line. To hear a pre-recorded commercial during a break at 10 minutes into your show, a pre-recorded commercial stored on a computer hooked up to a channel of the mixer would have to be played back and the engineer would have to open or increase the level of the potentiometer on the mixer for the commercial to be heard. With the mixer, you have placed that sound at a specific location - at the loudness you prefer.
Do you need a mixer for your Podcast? Analog mixers are good for live projects. If your show will normally consist of two or more people talking, you very well might. The mixer allows you (or an engineer) to make sure the loudness is balanced during conversation and the final audio is consistent. Plus, if you're going to integrate phone calls into your Podcast you will definitely want a way to control the inconsistencies of phone audio. For a better idea of analog mixer prices, see: Audio Mixers For Creating Podcasts or Internet Radio.
New School - Digital
Digital mixing is exceptional for mixing audio in a static project - one that does not require the Podcast or radio show to be broadcast or heard in real time. Free audio recording programs like Audacity, Kristal Audio Engine, and Wavosaur (all free) allow you to build your final audio by introducing layers of sound. Drag and drop a show open, record or import an audio file with your voice, etc. You can adjust each clip loudness and determine it's location.
Audio recording software will usually also have digitally created effects and filters you can add to parts or all of your project. As a veteran of editing beginning with reel-to-reel tape and razor blades in the beginning to the present, I can't impress upon you how extraordinary digital editing is! And: it's really not that hard to learn.
Audio Editors that are Mixers
Although mixing can be done with any multi-track recorder like Audacity, there are some program designed to assist in mixing. One program is WildVoice which allows you to record your voice and mix in music and sound effects while you're talking. You can then edit and "mix" your audio into one final sound file. WildVoice is free. See more on WildVoice here.
PodProducer is a handy little application which is very easy to use and surprisingly robust. It employs various features which allow for easy recording and audio mixing along with some nice enhancements which can make a Podcast sound pretty professional. See more about PodProducer.
