In fact, they date back to the 18th century according to Emily Post. It seems that a father did not like his daughter's choice of husband and refused to give her dowry. Friends of the groom's were insulted by the brides father. In support of the groom, they collected "assets" among themselves, "showered" the bride with them and the marriage happened. Today's showers have evolved over time. but there are still some "rules" or guidelines most brides follow. The bride's maid of honor is the usual hostess for a wedding shower, but any bridesmaid, friend of the bride's mother or groom's mother or distant relative can be hostess. It is usual that no member of the bride's immediate family (nor the groom's for that matter) host a shower so that it doesn't look like they are soliciting gifts for the bride. This also extends to the bride. It is never OK for the bride to throw a shower for herself.
With all of the talk of 'themed" weddings, potential hostesses wonder if they need to plan a shower in keeping with the theme of the wedding. The answer is no. The shower simply celebrates the upcoming marriage. The closest events come to a "theme" is kind of gifts guests bring to the shower, such as spa items, kitchen items, or lingerie. Some showers are for couples. Gift items for those should appeal to both genders like "stock the bar" parties, tools, outdoor equipment, etc.
Who should come? Only guests invited to the wedding should be invited to the shower. The only exception is for a workplace shower where not all work colleagues may be invited to the wedding. Nor is it necessary or even feasible to invite to a shower, every female who is invited to the wedding.
Including gift registry information on the shower invitation is acceptable (but this is the only place). Brides should open all shower gifts at the shower and thank each person as the gift is opened. Each gift must be followed by a handwritten thank you note. If multiple showers are given for the bride, guests attended more than one shower need only to give one gift. If a guest cannot attend a shower, she should not feel obligated to send a gift.