Is the idol target of worship or is God the target? Is the idol used only to get nearer to God? If yes how can an idol bring us nearer to God?
No, idols are just a mean to grow inner faith, devotion and attachment to God. So the Garuda Puran states:
na kāśṭhe vidyate devo na śilāyam na mrutsuca
bhāve hi vasate devastasmādbhāvo hi kāraṇam [GP - 2.28.11]
Deva (god) is neither present in the wood nor in the stone, nor in the soil (i.e. idols made of these things). He resides only in the emotion or feelings (bhava), hence bhava (devotion / emotion) is the only reason.
But it's difficult for an embodied being with a form to concentrate and worship the formless:
For those whose minds are attached to the unmanifested, impersonal feature of the Supreme, advancement is very troublesome. To make progress in that discipline is always difﬁcult for those who are embodied. [BG - 12.5]
Hence, in Hinduism there is also the provision for worshiping idols.