user7225: I want to avoid using the big USB B connector to program my Arduino Mega. Since there are those RX TX Pins I thought, it would be possible, to connect my USB - Wires from my PC directly to those RX TX Pins. D+ -> TX and D- RX. This Post on Arduino Stackexchange says, those wires are just connected, while this post says the exact opposite. 
I'm confused. Is it possible or would I damage my board when connecting USB wireq to RX / TX pins on my Arduino?

user6959: The first answer that you cite that you say says it is possible is in fact not talking about  what you are referring to  - it is discussing the use of the RX & TX pins directly when they are not being used for USB connection.
The second answer you cite is technically incorrect but "correct enough" in the context.
The answer to your question  is "It can be done but it requires a different boot loader, its slower, its technically inferior, it's less standard, it requires a degree of understanding (on someone's part) to get it going, and it can be done 
It IS possible to access USB directly from digital I/O pins. As TX & RX are also standard digital I/O pins they could be used for this purpose BUT you need dedicated bootloader code to do this and it has inferior performance if a USB to serial converter is available. The eg ADAFruit Arduino Trinket uses this system.  

Here is how the (or a)  ADAFRUIT Trinket Bootloader works. (The Trinket is not mentioned here but I'm told that this is what it uses. If not, it works in functionally the same manner). 
This is in turn based on the USBaspLoader
A (very) simple "more or less Arduino compatible" development board which can use this system is Metaboard. It's circuit diagram is shown below. As can be seen - it uses an ATMega168 and the USB connection is made to two standard digital I/O pins.

"Metaboard" using ATMega168 

note USB connection to pins PD2 & PD4 - NOT via RXD on PD0 and TXD on PD1 via a serial to USB converter, as is more usual. 


AND another version:  AVRUSBBOOT


And again
BootloadHID   They say  -

BootloadHID is a USB boot loader for AVR microcontrollers. The uploader tool requires no kernel level driver on Windows and can therefore be run without installing any DLLs.
BootloadHID is a USB boot loader for AVR microcontrollers. It can be used on all AVRs with at least 2 kB of boot loader section, e.g. the popular ATMega8. The firmware is flashed into the upper 2 kB of the flash memory and takes control immediately after reset. If a certain hardware condition is met (this condition can be configured, e.g. a jumper), the boot loader waits for data on the USB interface and loads it into the remaining part of the flash memory. If the condition is not met, control is passed to the loaded firmware.
This boot loader is similar to Thomas Fischl’s avrusbboot, except that it is built on top of the HID device class. This implementation is more user friendly on Windows, since no kernel level drivers need to be installed.


7225: <p>I want to avoid using the big USB B connector to program my Arduino Mega. Since there are those RX TX Pins I thought, it would be possible, to connect my USB - Wires from my PC directly to those RX TX Pins. <code>D+ -&gt; TX and D- RX</code>. <a href="https://arduino.stackexchange.com/questions/3772/can-tx-and-rx-pins-on-the-uno-be-used-like-regular-digital-pins">This Post on Arduino Stackexchange</a> says, those wires are just connected, while <a href="https://arduino.stackexchange.com/questions/4652/usb-d-d-to-arduino-mega-rx-tx">this post</a> says the exact opposite. </p>

<p>I'm confused. Is it possible or would I damage my board when connecting USB wireq to RX / TX pins on my Arduino?</p>
 6959: <p>The first answer that you cite that you say says it is possible is in fact not talking about  what you are referring to  - it is discussing the use of the RX &amp; TX pins directly when they are not being used for USB connection.</p>

<p>The second answer you cite is technically incorrect but "correct enough" in the context.</p>

<p>The answer to your question  is "It can be done but it requires a different boot loader, its slower, its technically inferior, it's less standard, it requires a degree of understanding (on someone's part) to get it going, and <strong>it can be done</strong> </p>

<p>It IS possible to access USB directly from digital I/O pins. As TX &amp; RX are also standard digital I/O pins they could be used for this purpose BUT you need dedicated bootloader code to do this and it has inferior performance if a USB to serial converter is available. The eg ADAFruit Arduino Trinket uses this system.  </p>

<hr>

<p>Here is how the (or a)  <a href="http://www.embedded-creations.com/projects/attiny85-usb-bootloader-overview/details" rel="nofollow noreferrer"><strong>ADAFRUIT Trinket Bootloader</strong></a> works. (The Trinket is not mentioned here but I'm told that this is what it uses. If not, it works in functionally the same manner). </p>

<p>This is in turn based on the <a href="http://www.obdev.at/products/vusb/usbasploader.html" rel="nofollow noreferrer"><strong>USBaspLoader</strong></a></p>

<p>A (very) simple "more or less Arduino compatible" development board which can use this system is <a href="https://metalab.at/wiki/Metaboard" rel="nofollow noreferrer">Metaboard</a>. It's circuit diagram is shown below. As can be seen - it uses an ATMega168 and the USB connection is made to two standard digital I/O pins.</p>

<p><img src="https://i.stack.imgur.com/ScLWt.jpg" alt="enter image description here"></p>

<p><strong>"Metaboard" using ATMega168</strong> </p>

<ul>
<li>note USB connection to pins PD2 &amp; PD4 - NOT via RXD on PD0 and TXD on PD1 via a serial to USB converter, as is more usual. </li>
</ul>

<hr>

<p>AND another version:  <a href="http://www.fischl.de/avrusbboot/" rel="nofollow noreferrer"><strong>AVRUSBBOOT</strong></a></p>

<p><img src="https://i.stack.imgur.com/uMRel.png" alt="enter image description here"></p>

<hr>

<p>And <strong>again</strong></p>

<p><a href="http://www.obdev.at/products/vusb/bootloadhid.html" rel="nofollow noreferrer"><strong>BootloadHID</strong></a>   They say  -</p>

<ul>
<li><p>BootloadHID is a USB boot loader for AVR microcontrollers. The uploader tool requires no kernel level driver on Windows and can therefore be run without installing any DLLs.</p>

<p>BootloadHID is a USB boot loader for AVR microcontrollers. It can be used on all AVRs with at least 2 kB of boot loader section, e.g. the popular ATMega8. The firmware is flashed into the upper 2 kB of the flash memory and takes control immediately after reset. If a certain hardware condition is met (this condition can be configured, e.g. a jumper), the boot loader waits for data on the USB interface and loads it into the remaining part of the flash memory. If the condition is not met, control is passed to the loaded firmware.</p>

<p>This boot loader is similar to Thomas Fischl’s avrusbboot, except that it is built on top of the HID device class. This implementation is more user friendly on Windows, since no kernel level drivers need to be installed.</p></li>
</ul>
