In the nlme package there are two functions for fitting linear models (lme and gls). What are the differences between them in terms of the types of models that can be fit, and the fitting process? What is the design rational for having two functions to fit linear mixed models where most other systems (e.g. SAS SPSS) only have one? Update: Added bounty. Interested to know differences in the fitting process, and the rational.

gls() vs. lme() in the nlme package

In the nlme package there are two functions for fitting linear models (lme and gls).

What are the differences between them in terms of the types of models that can be fit, and the fitting process?
What is the design rational for having two functions to fit linear mixed models where most other systems (e.g. SAS SPSS) only have one?

Update: Added bounty. Interested to know differences in the fitting process, and the rational.