How to connect/interface a USB cam (web cam) to a micro-controller?

You need to use USB to UART communication protocol. It is a very detailed module and you need to spend some time on it to understand

USB to UART comm protocol to allow them see each other

When using PIC for this purpose, it is necessary to select a PIC microcontroller with Fs Device/Host/OTG Capability like :-

PIC24 and dsPIC33 Families

PIC32MZ Family

PIC32MX2/4/5/6/7 Families

It is also necessary to know programming languages(Embedded C,Assembly or Hex if you can) to program the PIC and the working of usb devices and instructions on how to make the webcam interact with the PIC.

soit en utilsant un PIC qui possède un module USB hardware (PIC18F par exp) 

ou bien en utilisant un convertisseur USB /SPI ou UART..

convert USB to serial interface and connect to microcontroller through USART.

using pic microcontroller or mat lab program.

or interfacing with a arm controller

FOR ME I USE APPLICATION C# AND INTERFACE WITH  MICROCONTROLLER BY PROTOCOLE UART

You need to use USB to UART communication protocol

using PIC microcontrollers and programming using C++