Sometimes people may find their hard-drive space being progressively used up, and there are many reasons why this could happen; however, though people will generally look in their documents, movies, music, and other standard home directory folders to find large files, they may not touch the Library folder.
What is the user Library?
The Library folder is a location for applications and system processes to store settings and shared resources. For instance, you can put a Web plug-in in the "Internet Plug-Ins" folder and it will be used by any Web browser. There is no need to have a copy in separate folders for Safari, Firefox, or Opera. Other shared resource folders include ones like "Sounds" or "Speech," which are available for use by any application or system process that the user is running.
In addition to common-use directories, individual applications may make their own resource folders in the Library, so you will see folders like "Mail" or "iTunes" that have their own structure for holding application-specific plug-ins, add-ons, or other resources. Application-specific folders may be either the application name (for example, "iTunes") or the name of the developing company (for example, "Adobe" or "Mozilla").
Not only are static resources like plug-ins, application templates, and sounds placed in the Library, but many dynamic resources are also stored there. For instance, user caches to help quickly launch programs and revolving e-mail accounts are placed in the user's Library. This means that over time it will grow and shrink, depending on the applications used.
Risks from editing the Library
Because the files in the Library are accessed regularly by programs, tampering with them may cause odd program behavior so people generally avoid editing or removing them. Keep in mind that the Library started out with practically nothing in it besides some template files and folders, and it grew from use.
This means that though you may revert some settings by removing user Library files, you can usually restore these items by running your applications and customizing your settings again, and installing your custom templates, sounds, plug-ins, and other application and system enhancements. Generally you will not break an application's functionality by removing items from your user Library; the items important for application function are usually stored in the global Library folder.
A good rule of thumb to follow is if you use a specific application, then do not remove items for it in the Library. I have some GPS software with a large 500MB map of the U.S. stored in its "Application Support" folder. Since I use this item, I will not remove the large file. Doing so will not hurt anything, but will require me to reinstall and reconfigure the map.
Likely culprits of disk use
When looking at the items in the Library that are responsible for using up space, the main culprits are usually the Mail-related folders, user caches, and the "Application Support" folder. Others may include the Fonts, Audio, Sounds, and similar folders that are built to be accessed by multiple applications.
Depending on how many e-mail accounts you get, and how rules are managed, the e-mail folders can get quite large. I have a habit of keeping all my old e-mails, and as a result my "Mail" folder is several gigabytes in size. Not only are there the e-mail mailboxes, but also the "Mail Downloads" folder where downloaded attachments are stored, which can likewise be huge if you do not discard e-mails with large attachments.
The Caches folder holds fairly obscure documents that you will not be able to open or otherwise interact with. Generally a cache is a database (many times in a binary format to increase performance and reduce size) where last-opened or frequently used items are kept. Depending on how many applications are used and how they are coded to handle caches, the cache files can get quite large. You may notice Safari's cache can sometimes be several gigabytes in size, followed by other applications that need high performance with handling images such as Google Earth.
Many times cache sizes can be set in the program, but other times their sizes are based on the use of the program (if you only use Safari for viewing HTML-only Web sites that contain no images, then the cache size will be relatively small).
Lastly, is the "Application Support" folder, where applications will store frequently used items including license agreements, documentation, scripts, configurations, installation logs, and some plug-ins. The contents of this folder will grow based on how many applications, and also different versions of applications you have run on your system. For instance, if you have used Adobe Illustrator for years, you may find application support folders for Illustrator CS, CS2, CS3, and CS4 versions, which are no longer used but still reside in the Application Support folder. In my experiences I've found that games are the biggest users of the application support folder, and large map or level files containing sounds and detailed textures will be stored there.
Clearing up used space
To clear items in the user Library, all you need to do is first check to see if you need them, and then remove them if you do not. You will not want to do this for all items, but instead only for those that are using large amounts of space. Use a program like Grand Perspective, DaisyDisk, DiskInventory X, or WhatSize to catalog the files and display them based on relative size, and remove the large ones or large collections of unused ones if they are not needed.
Generally if you use a program regularly you should not touch its application-specific Library resources, but you can clear those for older versions of the same Library (for example, the older Adobe CS folders that are not applicable to the version you are using). Additionally, the files that are important for applications are stored in the global Library folder located in "Macintosh HD," so removing items in your user Library should not break applications. However, be sure to look up an item you are not sure about (do a Google search on it and its location) to ensure that removing them will not affect anything.
Another step beyond removing files by hand is to use a cleaning program such as OnyX, MacKeeper, or CleanMyMac to remove caches and other temporary items from the user account. These programs usually have a "user" section where they have options to select caches and other temporary items for removal.