The term cleanser refers to a product that cleans or removes dirt or other substances. A cleanser could be a detergent, and there are many types of cleansers that are produced with a specific objective or focus. For instance a degreaser or carburetor cleanser used in automotive mechanics for cleaning certain engine and car parts.
Other varieties include the ones used in cosmetology and dermatology or skin care. In this case, a cleanser is a facial care product that is used to remove make-up, dead skin cells, oil, dirt, and other types of pollutants from the skin of the face. This helps to unclog pores and prevent skin conditions such as acne. A cleanser is the first step in a skin care regimen and can be used in addition of a toner and moisturizer, following cleansing.
Using a cleanser designated for the facial skin to remove dirt is considered to be a better alternative to bar soap or another form of skin cleanser not specifically formulated for the face for the following reasons:

Bar soap has an alkaline pH (in the area of 9 to 10), and the skin's surface pH is on average 4.7. This means that soap can change the balance present in the skin to favor the overgrowth of some types of bacteria, increasing acne. In order to maintain a healthy pH balance and skin health, your skin must sit on the proper pH level.
Bar cleansers have thickeners that allow them to assume a bar shape. These thickeners can clog pores, leading to acne.
Using bar soap on the face can remove natural oils from the skin that form a barrier against water loss. This causes the sebaceous glands to subsequently overproduce oil, a condition known as reactive seborrhoea, which will lead to clogged pores. In order to prevent drying out the skin, many cleansers incorporate moisturizers.