hello everyone~ i am a beginner of ros , nowadays there are some questions frustrate me , most about robot navigation & gazebo.

the question is i want to write a robot (using urdf or sdf or xacro?) in gazebo and let it navigation themselves(by using gmapping , slam , locolization ros packages etc) , the first step to do this , i think i should make a robot description file and load it to gazebo , and "link" it to ROS , then at least i can get the odom information from ros message and use ros message like cmd_vel or teleop node to control the model move . but i found some question below:

1. i follow the tutorial "http://gazebosim.org/tutorials" and "http://sdformat.org/spec" to learn to write a robot model and added it to gazebo successful , but i don't understand the different between urdf & sdf & xacro , i saw some one add tag in urdf or xacro then it can also ues in gazebo - -, and i don't know where to find those xml format's specification , so if you know plz tell me ~

2.i also find a tutorial "http://guitarpenguin.is-programmer.com/posts/58100.html" that looks like this guy already added to irobot and could use cmdvel to control it move . but when i follow him ,add the <diffdrive_plugin> to my irobot sdf file ,it doesn't work well and when i spawn the robot the info in my terminal is :

[ INFO] [1420772905.328687303, 0.023000000]: waitForService: Service [/gazebo/setphysicsproperties] is now available. [ INFO] [1420772905.394811076, 0.086000000]: Physics dynamic reconfigure ready. [ INFO] [1420772910.103532720, 4.722000000]: DiffDrive missing , defaults is create [ INFO] [1420772910.103775476, 4.722000000]: Starting plugin DiffDrive(ns = create/)! [ WARN] [1420772910.103885161, 4.722000000]: DiffDrive(ns = create/): missing default is na [ INFO] [1420772910.106078458, 4.722000000]: DiffDrive(ns = create/): = create [ WARN] [1420772910.106351770, 4.722000000]: DiffDrive(ns = create/): missing default is odom [ WARN] [1420772910.106390086, 4.722000000]: DiffDrive(ns = create/): missing default is basefootprint [ WARN] [1420772910.106450532, 4.722000000]: DiffDrive(ns = create/): missing default is false [ WARN] [1420772910.106492915, 4.722000000]: DiffDrive(ns = create/): missing default is false [ WARN] [1420772910.106708018, 4.722000000]: DiffDrive(ns = create/): missing default is 0 [ WARN] [1420772910.106744029, 4.722000000]: DiffDrive(ns = create/): missing default is 5 [ WARN] [1420772910.107118573, 4.722000000]: DiffDrive(ns = create/): missing default is 1 [ WARN] [1420772910.107354021, 4.722000000]: GazeboRosDiffDrive Plugin (ns = ) missing , defaults to 1 [ INFO] [1420772910.108422069, 4.722000000]: DiffDrive(ns = create/): Try to subscribe to cmdvel! [ INFO] [1420772910.111925461, 4.722000000]: DiffDrive(ns = create/): Subscribe to cmd_vel! [ INFO] [1420772910.112774719, 4.722000000]: DiffDrive(ns = create/): Advertise odom on odom !

3.when i read the tutorial , it seems like that i need add & to my robot description xml file (whatever urdf ? xacro ? sdf ?) i don't know much about ros_controller or plugin , what it is ? what it is use for? and how should i use it ?

if you know some thing , or have the same frustrate with me , welcome to comment~ thx ~