Officially there are no female Space Marines. All of the original Space Marines were male, as far as we know, as were all the known Primarchs; this means that the process for creating new Marines was designed solely with males in mind and any consideration of changing this, without the utmost secrecy, would be considered the ultimate treason against the Emperor's design.
Some people have created their own chapters which include or exclusively field female Marines, such as the Fighting Tigers of Veda, and there is nothing stopping this being done if you put some effort into their background, just don't expect Games Workshop to ever start producing models.
Sisters of Battle are not Space Marines but a tribe of warrior-women brainwashed into being the personal guard of a corrupt Imperial governor who were subsequently redeemed into the service of the Emperor as the fighting force of the Ecclesiarchy.
This means that they are still human but receive some of the finest equipment available in the Imperium, e.g bolt weapons and power armour, hence the similarity to Space Marines.
As for what female Marines would be like - almost exactly the same as the men really.
Some people have created their own chapters which include or exclusively field female Marines, such as the Fighting Tigers of Veda, and there is nothing stopping this being done if you put some effort into their background, just don't expect Games Workshop to ever start producing models.
Sisters of Battle are not Space Marines but a tribe of warrior-women brainwashed into being the personal guard of a corrupt Imperial governor who were subsequently redeemed into the service of the Emperor as the fighting force of the Ecclesiarchy.
This means that they are still human but receive some of the finest equipment available in the Imperium, e.g bolt weapons and power armour, hence the similarity to Space Marines.
As for what female Marines would be like - almost exactly the same as the men really.