Well. I take example of Nikon AF-S DX NIKKOR 18-105mm f/3.5-5.6G 3.5 is the largest aperture at 18mm while the smallest aperture at 18mm was 22, thus aperture of 3.5 will allow more light to reach sensor under the same lighting conditions compared to aperture of 22. It will give faster shutter speed under same ISO setting to give the correct light exposure on the sensor.

The ISO is the sensitivity of the camera sensor to light. Higher ISO setting eg ISO 800 will be more sensitive to the amount of light reaching sensor compared to ISO 100. Relating ISO to aperture, say when you want to take a night shot at ISO 100 under normal room lighting, maybe you need a lens with the 'bigger' aperture of 1.8 while with the ISO 800, you will be able to capture the similarly exposed photo use a lens using a 'smaller' aperture of 5.6 By the way, in photography terms 1.4 is a larger aperture that 2.8 and the 'bigger' the number, it means smaller aperture.

As for DOF, it's the portion of a scene that appears sharp in the image. The larger aperture will give a shorter DOF, thus of blurred background or item in front of focused subject. Smaller aperture will give a longer DOF.

Shutter speed: A fast shutter speed will be able to 'freeze' the fast moving object to make it appear sharp in the image but with a slower shutter speed, the same motion might be captured as blur 'moving' object in the image.

Hope the info helps and not confuse you more...

Cheers,
William