See the "beware double meanings" discussion on the lesson home page; "la chatte" means female cat but is also a crude slang word meaning something like "❤❤❤❤❤", so be careful when you use it.
chatte is referring to a female cat. When it is a male, it is known as un chat :)
As Sitesurf has posted you use the masculine form when you don't know the gender.
Even if you know that a room full of cats, people, whatever are all female, if there is only one male present in the group then you use the masculine form.
In French there is no form of chat that is includes both genders so you have to select either masculine or feminine form. French culture stipulates that it be masculine unless there is a total feminine feature being described.
If you use chattes to refer to a room full of cats any listener will assume you have good reason to believe that they are all female.