Moving from Photoshop to Lightroom
Sailor Blue has the right information, but the terminology may be just a little confusing. Yes, Lightroom will detect whether or not you have an Adobe editor (Photoshop or Elements) installed on your computer and will set that up as your primary external editor.
You can set the preferences for your external editors in Edit>Preferences>External Editors tab.
RIGHT clicking on the thumbnail of the image you want to take to Photoshop will bring up a context menu that includes "Edit In" (or using Photo>Edit In in the menu system) - from there, you should see Photoshop as a choice. Other plugins (Nik products, for example) can be set up as external editors as well and they would also show up on the list.
Sailor Blue is quite correct that using File>Save in Photoshop will automatically save your image back to Lightroom as either a TIF or PSD (you make the choice in the preferences set for your external editor) in all its layered glory, but showing you a thumbnail that you can edit further in the Develop module. He is also correct that, should you want to take the image back to Photoshop a second time, you will be asked if you want to edit the original (minus any further changes you have made in Lightroom) or to edit a copy (a flattened file which will get a unique name, but will reflect any further changes you have made in Lightroom).