A sweetheart table is a table for two - just the bride and groom. Some couples know right off the bat that they don't want to be "put on stage" so this is not the option for them, but there are some benefits of having a sweetheart table. If you have a big bridal party, you don't have to worry about fitting everyone at one huge head table and their plus ones can sit with them at another table. When you are done with your salad course and entree course you are most likely going to get up to greet your guests table by table, so you are not leaving any of your "tablemates" behind and guests will actually come to your table and say hi as well!
If you are having a small bridal party and the sweetheart table is not your choice, you can choose to sit at a table with just your family - brothers, sisters, parents. Also depends on how big your family is of course! This table could be round, square or a long table. You can also make a table long enough to accommodate a larger family if you so chose. You may or may not get up and greet your guests in between courses, so keep that in mind as well. If you do not do a receiving line after the ceremony, it is custom for the couple to go and say hi to their guests.
Depending on the size of your bridal party (and their dates) you may choose to have a head table with your bridal party. These tables can be large, long tables as well with your guests on both sides. It is not very popular now-a-days to have all of your head table sitting on one side, and it is so much better to have someone across from you to chat with! And it really is best to have your bridal party sit with their significant others or dates so count them in when figure out size!
Take a look at some sweetheart tables and head table options from weddings we have done over the years. If you are still unsure - ask your bridal party what they think. And you may change your mind once you really start working on the floor plan!