Fun with ICC Profiles... Okay, "fun" is a relative term...
The question I'm asked most often is, "Do these socks make me look fat?" Another question that I'm often asked, but is not nearly as weird is, "What's the difference between assigning, tagging, converting and embedding an ICC profile in Photoshop?"
I'm glad you asked. Here is the answer....
Assigning a Profile
When you assign a profile in Photoshop, you are telling the program the meaning of the RGB or CMYK values. For example, you are telling Photoshop what specific shade of blue is represented by 46R 169G 232B. When you assign a profile, you do not change any RGB or CMYK values in the file, only the way in which those values are rendered to your display within Photoshop. To say it another way, you are changing the appearance of those values, not the values themselves. When you use the Assign Profile function in Photoshop (found under Edit) you can also remove a tagged profile by selecting Don't Color Manage This Document.
Tagging a file with an ICC profile is pretty much the same thing as Assigning a profile. Many people use the terms Assigning and Tagging interchangeably.
Converting lets you convert a file from it's profile space (or if the image is untagged, the current working space) to any other profiled color space. Here, the goal is to change the RGB values, not necessarily the color appearance. Typically a Convert to Profile function (also found under Edit) is used to convert a file from RGB to CMYK. The dialog box displays the source profile and allows you to select the destination profile along with other options like the rendering intent. Using Convert to Profile is always a better way to convert a file to CMYK because it gives you much more control than simply selecting Image - Mode - CMYK.
There are a few exceptions to embedding profiles. One instance is in a more closed-loop CMYK workflow. If all your images are destined for a standard press condition, you know that no one will be opening or editing your files any further, and you have converted all the files for that press condition (SWOP3 for example), then in a sense you have already tagged the file with the appropriate profile when you did the RGB-CMYK conversion.
Or suppose you are sending out a bunch of RGB images for a Web site. Microsoft Internet Explorer doesn't recognize ICC profiles (Safari and FireFox do), although Windows Vista does treat untagged images as sRGB. Embedding profiles also increases the file size to a degree, which is not a good thing when you want fast-loading images on a Web page. For now, it's best to just convert Web images to sRGB and call it good.
And no, those socks don't make you look fat.