What is the meaning of the "." (dot) in R? I'm just reading the book "R in a Nutshell".
And it seems as if I skipped the part where the "." as in "sample.formula" was explained. 
> sample.formula <- as.formula(y~x1+x2)

Is sample an object with a field formula as in other languages? And if so, how can I find out, what other fields/functions this object has? (Type declaration) 
EDIT:
I just found another confusing use of the ".":
> svm(formula = is_spam~., data = spambase.training)

(the dot between ~., )
 A: There are some exceptions (S3 method dispatch), but generally it is simply used as legibility aid, and as such has no special meaning.
A: The dot can be used as in normal name. It has however additional special interpretation. Suppose we have an object with specific class:
 a <- list(b=1)
 class(a) <- "myclass"

Now declare myfunction as standard generic in the following way:
 myfunction <- function(x,...) UseMethod("myfunction")

Now declare the function 
 myfunction.myclass <- function(x,...) x$b+1

Then the dot has special meaning. For all objects with class myclass calling 
 myfunction(a)

will actualy call function myfunction.myclass:
 > myfunction(a)
  [1] 2

This is used widely in R, the most apropriate example is function summary. Each class has its own summary function, so when you fit some model for example (which usually returns object with specific class), you need to invoke summary and it will call appropriate summary function for that specific model.
A: The dot in sample.formula doesn't separate sample from formula, other than visually. It is just a variable name. R variables names can consist of alphanumerics and dot (.) and underscore (_) with one exception. Here is the actual rule: 
"A syntactically valid name consists of letters, numbers and the dot or underline characters and starts with a letter or the dot not followed by a number. Names such as ".2way" are not valid, and neither are the reserved words."
The second case (i.e., the case of is_spam~.) is different and is explained above.
A: Look at the help page for ?formula with regard to . Here's the relevant bits:

There are two special interpretations of . in a formula. The usual one
  is in the context of a data argument of model fitting functions and
  means ‘all columns not otherwise in the formula’: see terms.formula.
  In the context of update.formula,  only, it means ‘what was previously
  in this part of the formula’.

Alternatively, the reshape and reshape2 packages use . and ... a bit differently (from ?cast):

There are a couple of special variables: "..." represents all other
  variables not used in the formula and "." represents no variable

