Use <ion-item> for something that is only non-clickable text.
Use <button ion-item> for something that can be clicked/tapped. Typically this element will also have a (click) handler.
Use <a ion-item> for when the item needs to contain a href.
By default, <button ion-item> and <a ion-item> will receive a right arrow icon on iOS to signal that tapping the item will reveal more information. To hide this icon, add the detail-none attribute to the item (eg: <button ion-item detail-none>). To add the icon when it is not displayed by default, add the detail-push attribute (eg: <ion-item detail-push>).
We can add some links to this, though, since it is buried in the docs.