The Antialiasing effect smoothes "jagged" edges on the rendered images. For example, it attenuates the step-like artefacts of simple straight lines.
The Bloom effect is a glow-like effect that makes bright areas of the image bleed into their darker surroundings.
The BrightnessContrast effect controls the brightness and contrast of the rendered image, just like on a monitor. The brightness refers to the overall lightness or darkness of the image. For eg, increasing the brightness makes every pixel in the image lighter by the same amount. The contrast controls the difference in brightness between objects in the image. Increasing the contrast makes light areas lighter and dark area in the in the image darker.
The HueSaturation effect lets you manipulate the colors in the rendered image. Shifting the hue lets you move each color by the same amount on the "color wheel" (think of the colors of a rainbow: red, orange, yellow, green, blue, purple... but cycling back to red). The saturation lets you control how dull or vibrant the colors of the rendered image should look. Reducing the saturation to the minimum makes the image grayscale.
The Vignette effect causes the periphery of the image to appear darker than its center (or lighter depending on the settings).
The AmbientOcclusion effect shades the points of each 3D object depending on how occluded (i.e. exposed to ambient light) they are.