There are three class methods you can use to build AssetBundles:
Building asset bundles is done through editor scripting. There is basic example of this in the scripting documentation for BuildPipeline.BuildAssetBundle.
For the sake of this example, copy and paste the script from the link above into a new C# script called ExportAssetBundles. This script should be placed in a folder named Editor, so that it works inside the Unity Editor.
Now in the Assets menu, you should see two new menu options.
For this example, you should create a new prefab. First create a new Cube by going to GameObject → Create Other → Cube, which will create a new cube in the Hierarchy View. Then drag the Cube from the Hierarchy View into the Project View, which will create a prefab of that object.
You should then right click the Cube prefab in the project window and select Build AssetBundle From Selection - Track dependencies. At this point you will be presented with a window to save the bundled asset. If you created a new folder called "AssetBundles" and saved the cube as Cube.unity3d, your project window will now look something like this.
At this point, you can move the AssetBundle Cube.unity3d elsewhere on your local storage, or upload it to a server of your choice.
Downloading Asset Bundles Example