A limiter is like compression only that it stops the signal volume going over a set level. It usually has a really fast attack time to stop transients from getting through, the limiter therefore can be used to squash signals closer and closer to 0dB without clipping, this can make a mix sound louder and this is done by reducing the dynamic range of the track. It is usually used at the end of the signal chain on mastering to bring the levels up to a level where the engineer, musician, artist or producer feels like it sounds right.
Limiters usually come with a fixed ratio of inf : 1 and with a attack time set to 0.00ms but as more limiters have been released more are getting more flexible in the option you can use to suit your mix, (as shown in the picture below)
Threshold - A level in dB at where the limiter is activated, anything above the threshold value will be flattened/ shaped to the same level.
Attack - How quickly the limiter reacts to when the signal is above the threshold level is set, or in the oxford limiters case, The ATTACK fader allows the attack time to be increased to achieve an improvement in the sonic qualities of the peak reduction process, by allowing peak transient events to escape hard gain reduction. Since the plug-in has internal headroom, these overshoot peaks are retained and are not clipped.
Release - This is how quickly the limiter reacts to incoming signals in volume, as the release settings on a compressor, there will be a sweet spot between too fast and too slow.
Output Trim - output trim will reduce the overall volume after it has been hit by the limiter, this is good for uploading music to Soundcloud, YouTube etc where the uploading process reduces the overall volume from 0db level.
They are other parameters within the limiter as these above are the main values to get you up and running, further more to this you can learn more about the oxford limiter by visiting the UAD website.