What are the main differences between R data files?

Rda is just a short name for RData. You can just save(), load(), attach(), etc. just like you do with RData.

Rds stores a single R object. Yet, beyond that simple explanation, there are several differences from a "standard" storage. Probably this R-manual Link to readRDS() function clarifies such distinctions sufficiently.

So, answering your questions:

• The difference is not about the compression, but serialization (See this page)
• Like shown in the manual page, you may wanna use it to restore a certain object with a different name, for instance.
• You may readRDS() and save(), or load() and saveRDS() selectively.

In addition to @KenM's answer, another important distinction is that, when loading in a saved object, you can assign the contents of an Rds file. Not so for Rda

> x <- 1:5> save(x, file="x.Rda")> saveRDS(x, file="x.Rds")> rm(x)## ASSIGN USING readRDS> new_x1 <- readRDS("x.Rds")> new_x1[1] 1 2 3 4 5## 'ASSIGN' USING load -- note the result> new_x2 <- load("x.Rda")loading in to  <environment: R_GlobalEnv> > new_x2[1] "x"# NOTE: `load()` simply returns the name of the objects loaded. Not the values. > x[1] 1 2 3 4 5