Let's start by talking copyright for a few minutes. From your blog to your virtual-environment creations you, the creator, have rights. You have the right to profit from your work (or the right to choose not to profit from it, if you like). Those rights are nothing without protection.
In almost every part of the world, while copyright laws disagree on some of the details, copyright is automatic when you create your work. The USA will require you to register your copyright before filing any infringement lawsuit, but short of a lawsuit you get some protections up-front.
Something to note, however, copyright does not apply to names, titles, ideas, slogans or short phrases. You should look into trademarks and patents for that. It also doesn't apply to Internet domain names, and you should refer to ICANN for those. From 1990, you're able to also copyright architecture, however, which is nice.
To defend your copyright, you should ensure there's a verifiable record of the date. Blogs mostly do that automatically, Second Life's content upload systems track the date as well, and publishers always include print and copyright dates.
Publication of a work isn't necessary to gain protection. Copyright vests in you the moment you create it, even if you never show it to anyone.
If there doesn't seem to be any other clear means, you can always try the "Poor Man's Proof" system, and send a copy to yourself by registered mail, then leave it unopened in your files in case you should ever need to prove the creation date. Registering your work through your national copyright organization is generally preferable to that, but isn't available in every country.
Most of us will never actually need to present that sort of proof in our lifetimes, thankfully.
Copyright basically grants the creator the sole rights to copy the work, or create derived works, or to grant others permission to do so, but for a limited amount of time only. Copyright can be bought and sold, licensed, given freely or inherited. As a creator you can choose to profit from your work, or to allow others to do so, to give out your work freely, or allow others to distribute or use your work under any lawful conditions you wish. When your copyright eventually expires, the work becomes 'public domain', and may be used by anyone, at any time, for any lawful purpose.
'Fair use' (or 'fair dealing') constitute a set of exemptions where people can have limited rights to your work without requiring your permission or license. Exactly what those exemptions are varies from country to country, and they're a bit involved to go into here, but you should be aware of them.
In Second Life, you should always pay particular attention to Next Owner Permissions, and you should take especial care to set them appropriately.
The three provided permissions are Modify, Copy and Transfer.
When an item is given or sold to any other person, those permissions will be in effect.
Modify allows the new owner to alter the item.
Copy allows the user to make as many copies of the item as they please.
Transfer allows the item to be given away or resold, at their option.
The system isn't perfect, and you should consider carefully what permissions you should set on an item.
Some combinations are useless for some kinds of items. A notecard, for example, cannot be read if it has neither Modify nor Copy permissions. A texture with Modify+Copy+Transfer permissions can be downloaded to your hard-drive.
Having Modify, Copy and Transfer permissions all set (known as 'full perms') is essentially the technical equivalent of making your Second Life creation public domain. Functionally you're surrendering all control over it once you've given a copy to someone else -- unless you have arranged some licensing or other contractual arrangement with them. This sort of thing needs to be watched very carefully, because the genie is damn hard to put back into the bottle.
Likewise you should watch out with Copy+Transfer -- people may not be able to modify the object, but a popular item can be in half of the user inventories in Second Life before the week is out.
Unfortunately, certain businesses (texture wholesalers, for example) need to use both these permissions at times as a part of the content that they sell. That's why they always sell them with conditions, and a license describing the usage they are allowing. As a responsible user, you should always pay attention to those usage licenses -- almost everyone will freely disclose the terms before they sell so you know what you're getting, and what your limitations will be. If someone breaks a license though, a seller can lose all their business. Genies and bottles.
Mixing assets with different permissions into a single item can generate confusing circumstances. Creators need to watch out that everything is set as it should be. Boy Lane's Cool Viewer has a bulk permissions editor (which is being folded into the official Second Life viewer as of 1.23), which helps simplify things a lot. However, due to the nature of communications between Second Life viewers and Second Life servers, it is possible that it may not work perfectly at times (just like when you texture an object and sometimes not every face gets updated with the new texture).
A proposed feature, VWR-8049, is intended to allow advanced content creators to set permissions-masks (as is done with files on computers) to set the permissions the creator wants to have at the time the asset is created, reducing some of the fiddly workload involved. Assuming it is properly and thoughtfully implemented and tested.
Know your rights, exercise them with care and judgement, and respect the rights of others. Those are the keys to protecting your works, online and offline. The system works best when the majority participate willingly, whether they are creators or consumers.