Currently there are 5 main marker types:
Orange with a central dot is a spot that you own and might be willing to share. You can lock these spots on the spot edit page, in order to prevent sharing them by mistake.
Red with a central lock outline is a spot that you own and are not willing to share. You can unlock these spots on the spot edit page, in order to share them.
Red with central people outlines is a spot that someone else shared with you. You cannot share these spots with anyone else and you can change only some parts of the information.
When a catch is associated with a spot, the interior of the spot changes to a fish outline.
When a marker is selected, it is outlined by a yellow ring.