This appears to be what you want, based on the example you gave:
sed -i 's/foo/bar/g' *
It is not recursive (it will not descend into subdirectories). For a nice solution replacing in selected files throughout a tree I would use find:
find . -name '*.html' -print -exec sed -i.bak 's/foo/bar/g' {} \;
The *.html is the expression that files must match, the .bak after the -i makes a copy of the original file, with a .bak extension (it can be any extension you like) and the g at the end of the sed expression tells sed to replace multiple copies on one line (rather than only the first one). The -print to find is a convenience to show which files were being matched. All this depends on the exact versions of these tools on your system.