If you are using more than one Mac, or have replaced your Mac with a new one, there are several ways you can migrate your account to the new system.
Conventionally, when you get a new system you are presented with the OS X Setup Assistant, which will guide you through migrating your data from an existing system or a backup.
In addition to Setup Assistant, you can use Apple's Migration Assistant tool at any time to transfer data from one Mac to another. This tool is especially useful for establishing your current user account on another system. Simply open the Migration Assistant tool on the destination system, and then follow its instructions for accessing your source Mac. Then you can check only the account you wish to transfer so its data and settings are integrated into the current setup of the destination system. This may be useful if, for instance, you are disposing of an older Mac and want to get all its users running on another more recently purchased one.
A final option for transferring user accounts to a new system is to do so manually. While backups and migration tools are the most convenient options, they might not always be usable. For instance, if you have deleted a user account from your system but preserved that account's data, and do not have a recent backup of it in other locations, you may need to restore the preserved data folder for the account to use.
The process for restoring an account manually takes two steps. The first is to copy the home folder for the account back to the Users directory on the desired system. To do this, you can select the home folder and press Command-C to copy, followed by Command-V (or use Shift-Option-Command-V to paste and preserve access permissions) in the Macintosh HD > Users directory to paste it.
If you have only a backup of the contents of the user's account (such as the Documents, Movies, and Music folders), then first create a folder in the Users directory and name it the same as the short name of the user you are going to recreate. Then copy all of your preserved data into this folder.
The second step is to open the Users & Groups system preferences and create a new user account. Be sure the short name you use for the account is the same as the home folder you have created in the Users directory. When you do this, OS X should ask whether you would like to use the existing home folder for the new user. Choose this and proceed with creating the user's account.
If you are not prompted with this request, then once the account is created, right-click it and choose the "Advanced Options..." feature. In here, click the button called "Choose..." next to the home directory field, and select the original folder you either copied or created for use as the user's home directory. Then click OK to save these changes.
With just these steps, you should be able to log into the account, but you may run into access errors that result in warnings, odd program function, or even a hang or two. This is because when copying the files or manually creating home directories, some important file permissions might get altered. Using the system's "Copy Exactly" feature should help prevent this, but it's not a guarantee. To ensure that no such issues come up, reset the user folder's permissions before logging into that user's account.
To do this, reboot and hold Command-R to load the OS X Recovery HD partition. After selecting your language, choose Terminal from the Utilities menu and run the command "resetpassword" (all one word, and lower-case) to launch the password reset tool. In this tool, select your hard drive, then the new user account, and then choose the option to reset home folder permissions and ACLs.
This routine should ensure that the home folder and its contents are properly accessible by that user account, so when finished, you should be able to reboot and log in to it without any errors.