The UG (haftungsbeschränkt), literally 'entrepreneurial company (with limited liability)', was introduced in 2008 to give German founders an ultra-cheap entry point to limited-liability structure. It can be incorporated with as little as €1 of share capital and uses the Musterprotokoll (template founding deed), keeping notary costs around €300.
The trade-off: the UG must allocate 25% of its annual profit to a statutory reserve until that reserve plus existing share capital reaches €25,000, at which point it can convert to a full GmbH. Most German pre-seed founders incorporate as a UG, and venture investors routinely require conversion to a GmbH as a closing condition at Series A.