PIC18F67J90 Clock Selection
Actually, you have more than two oscillator options..


  which one to select and what is the difference between FOSC and
  INTOSC?


The first thing you need to do is to select which type of oscillator you want to be your main device oscillator ("primary oscillator"). You said you want to use the internal oscillator. 

To do that we have to set some configuration bits at flash time. The bits that we're interested in are the FOSC&lt;2:0> bits from the CONFIG2L Configuration Register:


  FOSC&lt;2:0>: Oscillator Selection bits
  
  
  111 = ECPLL OSC1/OSC2 as primary; ECPLL oscillator with PLL enabled;    CLKO on RA6
  110 = EC OSC1/OSC2 as primary; external clock with FOSC/4 output
  101 = HSPLL OSC1/OSC2 as primary; high-speed crystal/resonator with    software PLL control
  100 = HS OSC1/OSC2 as primary; high-speed crystal/resonator
  011 = INTPLL1 internal oscillator block with software PLL control;    FOSC/4 output
  010 = INTIO1 internal oscillator block with FOSC/4 output on RA6 and    I/O on RA7
  001 = INTPLL2 internal oscillator block with software PLL control and    I/O on RA6 and RA7
  000 = INTIO2 internal oscillator block with I/O on RA6 and RA7
  


So you need to set the FOSC&lt;2:0> config bits to '000' to select the internal oscillator, with no PLL and no output of the clock to any external pins.



PIC18F67J90 has two internal oscillators, though:


an internal oscillator block which provides an
8 MHz clock (±2% accuracy): INTOSC
an INTRC source (approximately 31 kHz, stable over temperature
and VDD)


Obviously we want to select the faster oscillator (INTOSC). To do that we have to set the INTSRC bit from the OSCTUNE (Oscillator Tuning Register):


  INTSRC: Internal Oscillator Low-Frequency Source Select bit
  
  
  1 = 31.25 kHz device clock derived from 8 MHz INTOSC source    (divide-by-256 enabled)
  0 = 31 kHz device clock derived from the INTRC 31 kHz oscillator
  


So INTSRC = 1.



Next you'll need to set the speed of the internal oscillator.
For the Fast RC internal oscillator (INTOSC), the default clock speed is 4 MHz, but you can change it at runtime by modifying the IRCF&lt;2:0> bits from the OSCCON (Oscillator Control Register):


  IRCF&lt;2:0>: INTOSC Source Frequency Select bits*
  
  
  111 = 8 MHz (INTOSC drives clock directly)
  110 = 4 MHz (default)
  101 = 2 MHz
  100 = 1 MHz
  011 = 500 kHz
  010 = 250 kHz
  001 = 125 kHz
  000 = 31 kHz (from either INTOSC/256 or INTRC)
  
  
  *Modifying these bits will cause an immediate clock frequency
  switch if the internal oscillator is providing the device clocks.


So you'll need to set the IRCF&lt;2:0> bits to '111' if you want your internal oscillator to run at 8 MHz.

That should do it..

Note1: Fosc can also mean the frequency of the oscillator. Be aware that the system clock will be 4 times slower than provided Fosc (System clock = Fosc / 4). That means that if you select an oscillator of 8 MHz in frequency, the PIC will work at 2 MIPS. That's because, for a 8-bit PIC, it takes 4 clock cycles to execute one instruction.

Note2: As always, you'll find comprehensive information about this topic in the device's datasheet.