Notified bodies perform conformity assessment of products within the scope of European directives for which they are appointed. All notified bodies can be found in the NANDO database and have a unique NANDO number. If the directive your product falls under does not allow you to use Module A for conformity assessment, then you need a notified body. Notified bodies must act in an impartial, independent manner for the public good. All activities carried out by a notified body is confidential.
Notified bodies are audited by either a notifying authority or a national accreditation body. This audit is done against the ISO 17000 series. Which specific norm depends on the directive, for example for RCD 2013/53/EU, many bodies are audited and sometimes accredited against ISO 17065:2012. You are free to choose any notified body you wish from any EEA member state and once your product is assessed it is free to circulate in the entire EEA market.
Aside from providing conformity assessment notified bodies can also help with providing information about the directives for which they are appointed and the products that fall under its scope. For example, in RCD 2013/53/EU, in the case of a change to the recreational craft if the owner of the Craft does not have sufficient expertise to determine if this constitutes a major craft conversion they may enlist a notified body to assist with the determination of whether a conformity assessment is required for the change (it triggers a major craft conversion).
Almost all European directives provide for some kind of sectoral coordination group. In the case of RCD 2013/53/EU, this is the Recreational craft sectoral group (RSG). You can find them here: http://www.rsg.be/