Use  =  for immediate assignment.   Use := for delayed assignment.
For immediate assignment, the expression is evaluated at the time of
assignment.  For delayed assignment, no evaluation is done at the
time of assignment.


In[2]:= a=2

Out[2]= 2

In[3]:= x=a+b

Out[3]= 2 + b

In[4]:= b=6

Out[4]= 6

In[5]:= x

Out[5]= 8

In[6]:= x=a+b

Out[6]= 8

In[7]:= b=9

Out[7]= 9

In[8]:= x

Out[8]= 8

In[9]:= x:=a+b

In[10]:= x

Out[10]= 11

In[11]:= b=3

Out[11]= 3

In[12]:= x

Out[12]= 5

In[13]:= Quit