I am trying to understand the sha-256 algorithm from [FIPS 180-2](http://csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf). I understood the padding and parsing of the message string. However after that it states (page 15):

> 
> For $i = 1$ to $N$:  
> 
>  {
> 
> 
> 1. Prepare the message schedule, ${W\_t}$: 
>  $$ W\_t= \begin{cases} M\_t(i) & 0 \leq t \leq 15 \\
>  ROTL^1( W\_{t -3} \oplus W\_{t-8} \oplus W\_{t - 14} \oplus W\_{t-16} ) &
>  16 \leq t \leq 79 \end{cases} $$
>  ...
> 
> 
> 

But I am unable to understand what $W\_t$ is in the first place. Could someone please help me out?

I know it states on page 3:

> 
> $W\_t\qquad$ The $t$th $w$-bit word of the message schedule.
> 
> 
> 

But what is the message schedule and how is it generated?