The skin names are followed by [WIN] or [MAC] to show on which platform the skin was created.
This information can be useful, because Macs and PCs don't have the same fonts installed (for example, Lucida Grande is common on Macs, not on PCs), and the fonts aren't drawn exactly the same way on each platform (essentially because of a point size difference).
Don't worry, you can download a skin created on the other platform, it will work. Indeed, Coollector will do its best to make the conversion.
But be aware that the conversion between the platforms isn't a 100% safe process. The result might not look exactly like the original. In some cases, you might even have to adjust some fonts with the skin editor.