Summarized from The Baseball Encyclopedia for older information, and from various news stories for newer information:
Four baseball bats that were used in setting single-season home run records. From left to right: bat used by Babe Ruth to hit his 60th home run during the 1927 season, bat used by Roger Maris to hit his 61st home run during the 1961 season, bat used by Mark McGwire to hit his 70th home run during the 1998 season, and the bat used by Sammy Sosa for his 66th home run during the same season.
Williamson benefited from a very short outfield fence in his home ballpark, Lakeshore Park. During the park's previous years, balls hit over the fence in that park were ground-rule doubles, but in 1884 (its final year) they were credited as home runs. Williamson led the pace, but several of his Chicago teammates also topped the 20 HR mark that season. Of Williamson's total, 25 were hit at home, and only 2 on the road. Noticing the fluke involved, fans of the early 20th century were more impressed with Buck Freeman's total of 25 home runs in 1899 or Gavvy Cravath's 1915 total of 24.
Even with that relatively small quantity, and still pitching part-time, Ruth alone hit more home runs than did 10 of the 15 other major league clubs. The second-highest individual total was 12, by Gavvy Cravath of the Philadelphia Phillies. Ruth homered in every park in the league, the first time anyone had achieved that distinction. Ruth was a pitcher by trade, and the ultimate exception to the axiom that pitchers can't hit. Ruth had led the league with 11 in 1918, despite playing only 95 games, and still in the "dead-ball" era. By 1919, after the War, the materials for baseballs began to improve and became naturally "livelier".
Ruth hit just a few more home runs on the road (26) than he had the previous year (20), but he hit far more (29) in the Polo Grounds in New York (where the Yankees played at the time) than he had in Fenway Park (9) in Boston the year before, as he took full advantage of the nearby right field wall, although he also hit many long drives at the Polo Grounds. Of the other 15 major league clubs, only the Philadelphia Phillies exceeded Ruth's single-handed total, hitting 64 in their bandbox ballpark Baker Bowl. The second-highest individual total was the St. Louis Browns' George Sisler's 19. Ruth's major-league record slugging percentage (total bases / at bats) of .847 stood for the next 80 years.
59, by Ruth, New York (AL), 1921 (154 game schedule)
Ruth's slugging percentage was just .001 less than his record-setting average the previous year.
60, by Ruth, New York (AL), 1927 (154 game schedule)
Ruth hit more home runs in 1927 than any of the other seven American League teams. His closest rival was his teammate Lou Gehrig, who hit 47 homers that year.
61, by Roger Maris, New York (AL), 1961 (162 game schedule)
Pushing Maris that year was teammate Mickey Mantle; slowed by an injury late in the season, Mantle finished with 54. With the season being 8 games longer than in previous years – leading to the suggestion that official record keepers place an "asterisk" next to the record, many observers derided this situation as a major public relations gaffe by major league baseball.
After an epic battle between McGwire and Ken Griffey, Jr., who both got into the 50s in 1997, many expected the two to take on Maris in 1998. However, the player that competed for the record with McGwire in 1998 was Sammy Sosa of the Chicago Cubs, who propelled himself into the race with a record-setting 20 home runs that June. He would finish with 66 that season and actually led McGwire for approximately 45 minutes after hitting his 66th, until McGwire hit his own 66th, and four more in his final three games of the season. McGwire broke the old records in 144 games - fewer than even the old 154 game season. That removed season-length as a source of "asterisk" controversy, but McGwire's connection to the steroid scandal introduced a new call for asterisks on this and other records set in this era. On January 11, 2010 McGwire admitted using steroids during the 1998 season. McGwire claimed to use steroids to help heal an ailing body and denied they helped him to hit a baseball.
In part due to 9/11 terrorist attacks, the then-recently set record of only three years by McGwire, and Bonds' poor relationship with the media and some fans, Bonds' record setting was not as publicized as the previous chases. Bonds was initially chased closely by Sammy Sosa of the Chicago Cubs and Luis Gonzalez of the Arizona Diamondbacks, but Gonzalez faded late and finished with 57, while Sosa finished closer with 64 to become the first player to exceed 60 home runs in three separate seasons. McGwire was not a factor, in his final major league season, with the injuries that had plagued him for much of his career finally taking their toll, although he still hit at a pace that would have put him near 50 if he had played a full season. Bonds' slugging percentage of .863 broke the major league record set by Ruth in 1920. As happened with McGwire's record, Bonds' connection to steroids resulted in his being among the accomplishments questioned in the era.