[R] lm on group

[newbieRRRRR]

Hi all,
    
      I have a quick question about lm on group, say I have:

x <- 1:10
y <- x*3
buckets <- seq(0, 10, by=5)
g <- cut(x, buckets)
summary(lm(y ~ g - 1))

      Coefficients:
              Estimate Std. Error t value Pr(>|t|)    
g(0,5]       9.000      2.121   4.243  0.00283 ** 
g(5,10]   24.000      2.121  11.314 3.35e-06 ***

     What is it doing exactly? I guess the estimate is the mean of the y's
in each group. 

     How about other stats.. what do they exactly mean when we do lm on
groups? 


Thanks a lot!
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