I know that in Arduino Uno there is a serial to USB converter (ATmega8U2).
What actually this converter does?
It converts data signals between USB and TTL serial. 
To be able connect and program Arduino's MCU via serial port from PC, it must be used some kind of converter. Older PC had RS232 so RS232/TTL could be used. New PCs, notebooks, tablets have usually USB only so that's a reason to use USB/serial converter.
Budvar10:
It converts data signals between USB and TTL serial.To be able connect and program Arduino's MCU via serial port from PC, it must be used some kind of converter. Older PC had RS232 so RS232/TTL could be used. New PCs, notebooks, tablets have usually USB only so that's a reason to use USB/serial converter.
TTL is 5V ('1') and OV ('0')?
Yes.
EDIT: In reaction to post below, 'yes' was in simple meaning. I did not want to overload the noob mind (no offense to iSelector).
iSelector:
TTL is 5V ('1') and OV ('0')?
Well, no.
Actually, this is an incorrect usage of the term "TTL".
TTL actually has logic levels of 0 and about 3 V.
All the devices we use here are actually CMOS which is 0 and 5 V or 0 and 3.3 V if using that supply voltage. And in fact, USB itself uses 0 and 3.3 V logic levels even though it uses 5 V for power which is why the converter chips generally make available a low-current supply of 3.3 V as well.
Best refer to them as "USB to serial" converters. They translate between the complex USB protocol which incorporates two-way handshaking, and a basic asynchronous serial protocol in each direction - and some separate handshake lines.
And just to confuse you more, many of those converter chips actually output 0 and 3.3 V logic levels.
TTL actually has logic levels of 0 and about 3 V.
No. TTL has logic levels of 0.7V and about 3V.
The 8u2 (or 16u2) is another microcontroller, which supports native USB (that is, the chip can directly interface with USB * ). It is programmed to act as a USB-serial adapter, with it's serial pins connected to pins 0 and 1 on the '328p. It can be reprogrammed to do other things (though people rarely do this - it's kinda awkward), like act as a USB MIDI device, or emulate a keyboard/mouse, etc. Many Uno clones have a different serial adapter, like the CH340G or FT232R - these devices only work as USB-serial converters, and can't be reprogrammed. However, they do a perfectly fine job as serial adapters.
5v serial (often called "TTL" serial) or 3.3v serial (also often called TTL serial) are HIGH in idle state, and mark a 1 with a LOW.
RS232 voltage levels are +10~15v and -10~15v. Big difference. Unfortunately, TTL logic levels are often referred to as RS232.
When in doubt, verify voltage on idle serial TX line of adapter.
- In fact, devices that don't support USB can be made to act as USB devices using something called VUSB. This is a hackjob that makes a mockery of the USB standard, but it does work most of the time. It's what they use for the digispark, for example. It's flakier than real native USB, unsurprisingly.