There are three main permissions that help protect the work of each individual object or prim, Copy, Transfer and modify. Each one has its own specific set of declared rules that will help express your creative design with others on your terms.
The Copy permission allows others to copy your prim if they have already received it but if you have given the prim to another user and the objects permission has been set to no copy then once you have given or sold the prim to them then it is no longer have it, the other avatar does. Example: an object such as a lamp.
The transfer permission allows one avatar to give or sell an object to another and enables the new owner to transfer that object on to others if they do not change the already set permissions. Giving an item that has the permission to transfer but not copy will allow the object to be given/sold to the next person but you will not own that object any longer. Example: objects that you do not mind giving to another i.e. once transferred this is no longer in your personal inventory (if this has no copy attached).
The Modify permission lets the creator select if another user will be able to modify or change the creation you have made. By doing this you can keep the work you have done the way it was without it being altered in anyway. It is also quite useful to have this turned on if you are working in a group build and letting other group members add certain things that can enhance the build, especially if you are stuck on what to do and are in need of help. Example: this enabling someone else modify your creations.
Each prim that has been rezed by you (the creator within Second Life) can have its permissions changed to suit your own personal needs. This protects your work in the way that best suits your needs.