Water repellant and water-resistant are terms used to describe degree at which a material will be able to withstand being saturated by water. These two terms are often compared to the term "waterproof." Typically, water-resistant is the lowest rating, while a material that is water repellant is likely to be of greater use when more water exposure occurs. Something that is waterproof may remain undamaged by significant contact with water, but even here, there may be a limit to how much liquid exposure the item can take.
With a water-resistant fabric used for things like tents, blankets, or clothing, people can expect a small amount of protection from minimal and short contact with things like rain. The water-resistant fabrics usually have a tight weave, and some natural fabrics like close woven wool give good temporary protection from moisture. A person wearing a jacket that resists water might be out in a brief rainstorm without having the rain soak through to interior clothing. The great advantage of this type of material can be that even though the weave is tight, it still offers breathability. When people expect contact with water to be minimal, using water-resistant items may be a good choice.
A water repellant fabric is less breathable because it tends to feature a much tighter weave. It may also be sprayed with coating that better keeps moisture from fully saturating the material. This can really make a difference in selecting certain items. Those looking for tents, for example, would want to find ones that are water repellant or waterproof instead of water resistant to get better protection from things like rainstorms. Similarly, a raincoat designed specifically for inclement weather is probably more effective if it repels instead of just resists water.
Waterproof usually means it's very difficult for moisture to do anything more than accumulate on the outside of the fabric, although there are different degrees of waterproofing. Some products, especially electronics, begin to lose their ability to resist water soaking through if water pressure is high enough. In terms of selecting fabrics, things that are waterproof are usually best chosen when a high degree of water exposure is expected. Having to work or simply be out in a significant rain storm might indicate wearing fabrics like a plastic poncho to protect interior clothing and the body from getting wet.
There's always a trade-off when selecting fabrics with one of these labels. A water-resistant garment might be lightest to carry or wear. By producing tighter weaves, something may become water repellant, but it may be much warmer and not as comfortable to wear or use in all seasons. Waterproof things usually have a very tight weave that can protect the body or anything else covered, but that may be impractical for wear or use in any situations where water exposure is not expected to be significant.