Because each major developed and is run by different organizations, they each have different characteristics that set them apart. These involve the character of the courses used, the composition of the field, and other idiosyncrasies.
The Masters Tournament is the only major that is played at the same course every year (Augusta National), being the invitational tournament of that golf club. The Masters invites the smallest field of the majors, generally under 100 players (although, like all the majors, it now ensures entry for all golfers among the World's top 50 prior to the event), and is the only one of the four majors that does not use "alternates" to replace qualified players who do not enter the event (usually due to injury). Former champions have a lifetime invitation to compete, and also included in the field are the current champions of the major amateur championships and most of the previous year's PGA Tour winners (winners of Fall Series events or "alternate" events held opposite a high-profile tournament do not receive automatic invitations). The traditions of Augusta, such as the awarding of a green jacket to the champion, create a distinctive character for the tournament, as does the course itself, with its lack of rough but severely undulating fairways and greens, and punitive use of lakes and creeks on several key holes on the back nine.
The U.S. Open Championship is notorious for being played on difficult courses that have tight fairways, challenging greens, demanding pin positions and thick and high rough, placing a great premium on accuracy, especially with driving and approach play. The U.S. Open is rarely won with a score much under par (although it is often played around courses where the severity of the test might suggest par itself has been set at a punitively low level). The event is the championship of the USGA, and in having a very strict exempt qualifiers list - made up of recent major champions, professionals currently ranked high in the world rankings or on the previous year's money lists around the world, and leading amateurs from recent USGA events - about half of the 156-person field still enters the tournament through open qualification events, mostly held in the U.S. but also in Europe and Japan. The U.S. Open has no barrier to entry for either women or junior players, as long as they meet the entry criteria for the qualifying sections. As of 2009, however, no female golfer has yet qualified for the U.S. Open. Also, the U.S. Open has a unique playoff system in that it requires all tied players to play an additional full round of golf with sudden death if necessary. (The Open and PGA Championships use four and three-hole aggregate playoffs respectively, followed by sudden death if necessary, and most regular events as well as the Masters only have simple sudden death playoffs.)
The Open Championship is organized by The R&A, an offshoot of The Royal and Ancient Golf Club of St Andrews, and has always been played on a links-style course in Britain or Northern Ireland (all of the courses on the current planned "Rota" are in Scotland and England). It carries the prestige of being the original "Open" championship (although ironically the very first event was held only for British professionals) and is respected for maintaining the tradition of links play that dates back to the very invention of the game, in Scotland. This is typified by courses that, being coastal, tend to be flat and often very windswept, with the fairways cut through very high dune grass and gorse bushes that make up the "rough", and have very deep bunkers. Also, unlike the U.S. Open (and to a lesser extent the PGA Championship), the course is generally not "doctored" to make it more difficult, effectively making the typically variable weather the largest external influence on the field's score. As well as exempting from qualifying recent professional major and amateur champions and leading players from the world rankings, the R&A ensures that leading golfers from around the globe are given the chance to enter by holding qualifying events on all continents, as well as holding final qualifying events around the UK in the weeks prior to the main tournament. Several recent champions have been relatively unknown players who came through one of these qualifying routes. The champion receives (and has his name inscribed on the base of) the famous "Claret Jug", a trophy that dates back to 1872 (champions from 1860 until 1871 received instead a championship "belt", much like a champion professional boxer's belt nowadays) and the engraving of the champions' name on the trophy prior to them receiving it is, in itself, one of the traditions of the closing ceremony of the championship, as is the award of the "silver medal" to the leading amateur player to have made the cut to play the last 36 holes.
The PGA Championship is traditionally played at a parkland golf club in the United States, and the courses chosen tend to be as difficult (although not set up quite as punitively) as those chosen for the U.S. Open, with several, such as Oakland Hills, Oak Hill and Winged Foot, having hosted both. The PGA of America generally enters into a profit-sharing agreement with the host golf club. As well as inviting recent professional major champions and leading professionals from the world rankings (the PGA has tended in recent years to extend invitations to all players among the world's top 100) the PGA Championship field is completed by qualifiers held among members of the PGA of America, the organization of golf club and teaching professionals that are separate from the members of the PGA Tour. The PGA Championship is also the only one of the four majors to invite all winners of PGA Tour events in the year preceding the tournament. Amateur golfers would not normally be featured in the PGA, unless their success in other events had lifted them high enough in the world rankings (there is no qualifying section open to amateurs) or by winning one of the other major championships (allowing an amateur to play every major championship for the remainder of the year plus 5 years after the win). One particular feature of the PGA tends to be the testing weather conditions - not the wind and rain that often accompany an Open Championship held in the British Isles in July, but the high heat and humidity that characterize the American climate in August.