I'm mapping out and implementing some permissions settings for a site that mostly involve changes through the UI, but at times require some under-the-hood tinkering in SharePoint Designer. I'm a bit new to this and the documentation isn't always clear.
I am sure the article below will help you understand the relations between permissions, permission levels, User/Groups and Securable objects;
Basically a permission level is a set of individual fine grained permissions that are logically grouped together keeping a certain role in mind.
Permissions themselves are fixed but permission levels can be created/modified and SharePoint creates some permission level for us to get started.
Securable Objects (look aticle above for a fixed set of securable objects that Sharepoint has) are essentially given a perticular permission level to a particular user/group. If concept of permission level was not there, you would have to give individual permissions to sharepoint users/groups for secuarble objects! what a nightmare that would be!