[BioC] a question on creating model.matrix using limma

[Srinivas Iyyer]

Dear Dr. Smith and group, 

apologies for simple question. 

I have data with simple plan.  There are 8 samples (4
normal and 4 tumor). These tissue samples are from
different patients. No replicate experiments were
done. 
I wish to see genes differentially expressed between
Tumors and normals.

In such cases how can I create model.matrix


Following is taken from Limma manual:

design <- model.matrix(~ -1 +
factor(c(1,1,1,2,2,3,3,3)))


my question: 
 1. what is '~ -1' is for?
 2. Will this work for my study structure. That means
I have only two types of RNA 1. tumor  and 2. Normal. 
design <- model.matrix(~ -1 +
factor(c(1,1,1,1,2,2,2,2))).

Thanks
sri