using var and not var keyword in C#

using var and not var keyword in C#

i have some confusing about var and not var initialization.
var x=10;
x=12;
what is the difference between those initialization in C# ?
i knew var is keyword is used for following kind of ways.
When the type is complex to write, such as a LINQ query (the reason for
var in the first place) use var.
Thanks,
Siva