Most of the time when possible I give you the possibility to install as either Sims3Pack or Package. Pick up a format you rather use (poses can only be installed as Packages), and install it in your game:
If you use EA Launcher:
Sims3Packs Place the .sims3pack file in your folder My Documents/Electronic Arts/The Sims 3/Downloads Run the Launcher, click the Download panel on the left and select the files you want to install before pressing the Install button. Note: You do not need to keep the file in this folder, it can be removed safely once installed and the Launcher/game is closed.
Packages Place the .package file in your folder My Documents/Electronic Arts/The Sims 3/Mods/Packages If you do not have or do not know how to set up the Mods folder, I wrote a tutorial here. Note: If you have too many files in this folder you should consider merging them or it will make your game load and render slower. Aikea Guinea wrote a tutorial about it, see here.
If you use CC Magic:
This launcher manages both types, so you can place any .sims3pack and .package files you want in My Documents/Electronic Arts/The Sims 3/Downloads For those who don't know about it, you can find the installation here and a tutorial there.