One of the new fitu in ArcGIS 10 is the ability to insert files (attachments) into the layer. With attachments, the user can add information pertaining to that layer. Attachments almost similar to hyperlinks, but the user can insert more than one file (various types of file formats) into a geodatabase layer.

Files that are used as inserts (attachments) are stored in the geodatabase, and the file can be displayed in several ways, according to the file format that has been inserted, can also be opened via the web applications, ArcGIS Explorer or ArcReader.
Links between the layers (feature classes) and files that are inserted using a relationship class, so the ability to insert a file or edit it only in ArcEditor or ArcInfo license. While ArcView license can only be used to display the attachments only.
Here is a summary of steps to enable the attachments feature in ArcGIS 10, add / edit the file to be inserted and how to display them.
(There are also mini-tutorial in PDF format full screenshot. Download link available at the end of this post)
Attachments feature is enabled through the Catalog window (ArcCatalog). The trick right click feature class, select Attachments -> Create Attachments. Drag the result into the ArcMap display.
Next right click on the feature class in the TOC, select Edit Features -> Start Editing.
Then by using the arrows on the toolbar Edit Tool, select an individual feature you want to paste the file into it, then right click select Attributes.
Select Open Attachment Manager. In the Attachment Manager window that opens, click Add, browse to the location of the file you want to insert into the layer.

To display the results, simply click on individual and last feature using Identify button. To open the file, double click on the file attachment.

You can also display the results in the form of HTML attachments Pop ups, by first doing a little feature settings in the properties class. The trick, right-click the feature class, select Properties. Layer Properties from the open, move it to the HTML tab Pop-up. Perform the settings and then click Verify. Click Apply then click OK. The result can be viewed using the HTML Popup button as shown in the picture at the top of this post.
Attachments are stored in the geodatabase, geodatabase so you can share to others or made a package using the toolbar folder opened in ArcReader Publisher to automatically file attachments that are already included in it.