The term “wedding party” refers to the people that participate in the wedding ceremony. This group includes the bride and groom (obviously) and a marriage officiant, other participants may vary depending on the religion, local traditions, style of the wedding, etc. Let us make the list of the most common roles in the wedding party.
Best man. A best man is groom's right hand. He is involved in all stages of wedding planning. The best man takes care of the bachelor party and supports the groom throughout the wedding. Best man is often responsible for keeping the wedding rings safe until the ceremony.
Maid of honor. A maid of honor is the bride's equivalent of the best man, she is the bride's chief attendant who provides emotional and practical support. She is responsible for organizing the bridal shower and bachelorette party. The maid of honor is usually asked to help with the logistics of the event and take care of numerous details.
It is worth noting that these two roles can be played by either gender, so if the bride wants her guy best friend be her “maid of honor”, it's perfectly OK (if the groom is OK with it). Of course, in this case the term “man of honor” or gender-neutral terms “honor attendant” or “best person” will be more appropriate. The roles of best man and made of honor are typically played by a sibling or close friend. In some countries, honor attendants are the official witnesses for the marriage license.
Bridesmaids and groomsmen. Bridesmaids are the female attendants to the bride, and groomsmen are male attendants to the groom. It is up to the bride/groom to determine the number of attendants in their wedding party, but the number of bridesmaids and groomsmen is usually about the same. Bridesmaids and groomsmen typically participate in pre-wedding events and help the couple and honor attendants during the wedding when asked to.
The roles of bridesmaids and groomsmen are normally played by siblings, other relatives, and close friends. If the wedding party includes minors, they are called junior bridesmaids and junior groomsmen. They also get to attend pre-wedding events excluding bachelor and bachelorette parties that typically involve drinking, striptease and other activities inappropriate for minors.
Flower girl. A flower girl is a girl between the ages of 3 and 10 in a nice dress who scatters flower petals or carries a bouquet in front of the bride as she walks down the aisle. This role is typically played by a young relative: a younger sister, a cousin, a niece, etc.
Ring bearer. A ring bearer, also known as a page boy, is a young boy between the ages of 5 and 10 responsible for carrying the wedding rings. Ring bearers are usually younger brothers, nephews, or cousins.
Father of the bride. Sometimes the father of the bride is asked to walk her down the aisle. This tradition stems from an ancient custom of giving away the bride. However, some brides choose to walk through the aisle unaccompanied or ask an uncle/brother/cousin/best friend to “give them away” is their father is unavailable for some reason.
The list above is not mandatory. It's up to you to decide which roles and how many people your wedding party is going to include.