Building Tools
Land Editor
In world, you can use the land editor to reshape terrain, if you have permission. The surface can be selected and then raised, lowered, smoothed, or roughened by some amount.
If you have permission, you can also change the texture (images) for the surface, such as sand near the water, grass, or rocky mountain tops.
Plants (Flora) are not part of the land. They are objects that you place on the ground. There are plants in the Inventory Library, and there are many more for free or for sale.
Bull Dozer
This is a special mode of the Land Editor. Instead of selecting land, you run a small bulldozer over it.
2D Graphics
There is no 2D graphics editor inside Second Life. Instead, you use any one of the many editors available outside, such as the GIMP, Photoshop, or even MS Paint.
3D Modeling
See this 3D modeling guide for detailed information.