http://www9.georgetown.edu/faculty/bassr/heath/syllabuild/iguide/glaspell.html - Links to a critical discussion of Trifles.
http://itech.fgcu.edu/faculty/wohlpart/alra/glaspell.htm#Trifles - Links to a hypertext version of the play.
http://vccslitonline.cc.va.us/drama/theme.htm - Links to site which explains the vague concept known as "theme."
http://www.csustan.edu/english/reuben/pal/chap8/glaspell.html - Links to Glaspell's bio.
What all four of these sites have in common is that they are reliable; someone at each of these colleges or universities ( note .edu) is responsible for the content. The sites are well-maintained and fairly easy to navigate.