They are part of the EU and are part of the EU customs area, there is no practical difference with other EU territories regarding trade.
They are not part of Schengen (but EU citizens don't need any visa, it's mostly intended to curb illegal immigration from South America via Guiana) and have the ability to use different VAT rates from mainland France, but that's all.
They are not part of Schengen (but EU citizens don't need any visa, it's mostly intended to curb illegal immigration from South America via Guiana) and have the ability to use different VAT rates from mainland France, but that's all.