Tap on the Application and then tap on the Details button. The last field 'Catalog' on the Summary tab will tell your what Application Catalog the software is in. Maemo Extras is considered a safe Application Catalog. Even newbie should be OK using that software. There is no * Testing nor * Development software in the Maemo Extras catalog.

Maemo ExtrasTesting and Maemo Extras Development are totally separate Application Catalogs which aren't included on the base firmware release. You have to manually add those application catalogs to be able to install any software located there. Extras Testing software is generally getting close to release so they might be OK for an adventuresome newbie. You should consider Extras Testing as release candidates. There still may be problems with them. And unless you really know what you are doing, avoid Maemo Extras Development catalogue because you can protentially break a lot of things by install software from Development. Avoid it like the plague.