If you use cutom content (content like StaticMeshes, textures, sounds, etc. that didn't come with the game) you have to distribute it with your map-file. Someone who downloads your map also needs the assets that are used in it. There are two ways of packaging such content.
1. In a regular package.
If it is done like that the end-user has to install these files into the file-structure of his game. This poses a minor though avoidable annoyance (see #2). If you can avoid it - do so! You can make it easier for him by distributing your map and your packages in an archive which has the same strukture as the game folder so he just has to extract it into it, however there is another problem: The other problem is that your package will clutter up the directories of the end-user. If he happens to be a modder himself it can become difficult for him to tell retail packages (ones that came with the game) from custom-packages and it will be hard for him to figure out a) which content he is allowed to use, for which he needs permission and what is off-limits (based on the respective readmes that came with the maps that came with the content) and b) which packages he has to pass along with his own map.
The big advantage of this type of packages is that the content in them can be re-used for multiple projects but it has to be present just once. This saves space and maybe bandwidth if a player connects to a server that runs a map he doesn't have yet but he already has the needed packages as they came with another map. Keep in mind though, that you should never modify a once released package!
Note: It is not enough to just package content in a package. E.g. you import a texture, set its package and forget about it. Such content will be lost once you close the editor and it will not show up in-game if you play-test the map from inside the editor. You have to open the (newly created?) package in the appropriate browser and save it from there! This is not done automatically!
About package names: They must be absolutely unique! If you create a texture package called Textures.utx and someone else had the same glorious idea the packages will be overwritten once someone installs both your and the other dude's map! So pick unique names! Due to the way the engine handles packages the file-extension (e.g. .utx, .usx, etc.) is not taken into consideration. Having a Trollcave.utx and a Trollcave.usx causes problems! If you still want all of your packages (texture-packages, StaticMesh-packages, sound-packages, script-package, etc.) to bear the same name (e.g. the name of the map) add a pre- or suffix to the name, based on the content in them. In this example: TrollcaveTex.utx and TrollcaveSMs.usx.
If you plan to use the content you are about to package only once consider the second method.
2. In the "myLevel" package
If you place your content in the package "myLevel" (without the """. This is not a placeholder for the name of your map! It really is "myLevel") you not only do not have to but you must not save it from the broser! It will automatically be saved within your map-file once you save your map. There will not be a seperate package. You can access the myLevel package from the drop-down list of packages in the browser. If you need something from the myLevel package of another map you can just open that map in UnrealEd and then open your own map. The myLevel package of all the previously opened maps of this session will be completely added to your currently open map. Use whatever you need from the huge myLevel package now. Once you save the map only what is actually in use in the map will be saved in its myLevel package! This makes sense or else you would have the whole myLevel package of another map clogging up your own myLevel package for no reason. However...
Note: If you imported content to the myLevel package but you didn't use it in the map yet it will be lost once you close the editor regardless of whether you saved your map or not! So use what you want to keep!