If you have a look in the 'Audio MIDI setup' utility you see that it is possible to set the sampling rate (example 44100.0 Hz) and sample size (example 16bit). It is probably best to make these settings the same for all devices you're using. Preferably the same as the sampling rate and sample size you're recording at. In the setup described here this is: 44100.0 Hz (or 44.1 kHz) and 16 bits.
Any time one application passes audio to another and the sampling rate/ sample size settings are not the same between the two a conversion has to take place. This conversion uses system resources (processor capacity). Since we're trying to minimize the amount of processor time used so the critical processes have more 'breathing space' it's a good idea to avoid those conversions.
(The term for such a conversion is upsampling - to a higher sample rate - or downsampling - to a lower sampling rate - if I'm not mistaken.)
Sampling rate is also known as sampling frequency.
Sample size is also refered to as 'precision' or 'definition.