Nearly all of the functionality of Windows (or Mac) applications can be found in similar applications on Linux today; with the added-plus that many on Linux are free! Everything from Office [Excel, Word, Powerpoint, Access]; to Skype; to Photoshop, to even Quickbooks has Linux open source equivalents.
But there may be cases where you want to run a native Windows app on your Linux pc. That's where Wine comes in. Wine is an app that provides Windows APIs that your Windows programs use to run. There are a very large set of Windows apps that have been confirmed to run in Wine. The current list is available here.