Natal charts are divided into twelve sections called "houses," which indicate the different areas of interest and activity in your life. How those houses are divided up depends on the house system used by you or your astrologer. There are at least eight major house systems in use throughout the world, but the three most commonly used in the United States are Placidus, Koch (pronounced "coke"), and Equal. These three systems result in the same Ascendant (Rising Sign) but have different Midheavens and house cusps.
The Placidian system is named after its 17th century inventor, Placidus de Titus. It is used by at least 80% of astrologers in the U.S. because it is the most widely known system and its house tables are readily available in most areas.
The Koch system is the second most commonly used house system in the U.S. It originated in Germany and is named for its first publisher, Dr. Walter Koch. Both Placidus and Koch systems result in the same Ascendant which is always the cusp of the first house.
The Midheaven is always the cusp of the tenth house, but each system results in a different sign (or the same sign at a different degree) on the Midheaven. Placidus and Koch are "unequal" house systems which can result in interceptions. An interception occurs when two opposite signs are each on the cusp of two houses, while another two opposite signs are not on any house cusps at all and are instead inside of houses with neighboring signs on their cusps. But in any given chart, the signs intercepted by Koch are not necessarily the same ones that would be intercepted using Placidus.
The Equal house system is just what its name implies. All the houses have the same degree on each cusp, and there are no interceptions. Equal house systems are especially useful for people born in the northernmost and southernmost latitudes, and this system is the most popular one in use outside of the United States.
No given house system is better than another, only different. Astrologers select the systems which they feel work best for them and for their clients. They also select systems which best suit the kinds of astrology they practice, giving them the desired interpretive results. For example, those who practice astropsychology are often inclined to use Placidus, and those who practice Uranian astrology are more likely to select Koch.
If you are unsure which system is best for you, try both, or try one of the other house systems, and see which produces the most reliable results. That's the best house system for you.