A dictionary is a source file that once compiled, linked into a library or executable and loaded into a process will give to CINT all the information its need about a type or variable. A dictionary is essential for a C++ to entity to useable from a CINT script or to be Streamed in or out of a ROOT file.
You have 3 options to create a dictionary.
Use ACLiC to generate the dictionary for everything defined in a source or header file:
root  .L myScript.cxx+
or (from compiled code):
gROOT->ProcessLine(".L myScript.cxx+")To generate the dictionary for a single class or for a class template instance use (with ROOT v5.27/06 and above):
gInterpreter->GenerateDictionary("vector<Track>","Track.h;vector"); gInterpreter->GenerateDictionary("myclass","myheader.h");To generate the dictionary from a Makefile, you can use the following rule to generate a dictionary for the headers $(HEADERS) and a library containing the dictionary and the compiled $(SOURCES):
MyDict.cxx: $(HEADERS) Linkdef.h [TAB] rootcint -f $@ -c $(CXXFLAGS) -p $^ libMyLib.so: MyDict.cxx $(SOURCES) [TAB] g++ -shared -o$@ `root-config --ldflags` $(CXXFLAGS) -I$(ROOTSYS)/include $^
See Interacting with Shared Libraries: rootcint for more details.