What is the Ubuntu equivalent of the Windows Start Menu?
Equivalent of windows start menu in Ubuntu is Dash. Press Super key (AKA 
key on a Windows-style keyboard) on the keyboard to open up dash. There you can search for installed applications, document, directories and also you can search for online materials (like games, musics, news, etc).
You can directly access the application lens by pressing Super+A (ie: + A). Through that you can access installed, recently used applications.
Unity, the default UI since Ubuntu 11.04, has this wonderful Dash desktop search utility. You can access Dash by hitting the Super key (aka key in Windows keyboard).
At the bottom of the Dash, click the application lens among others, as shown in the snapshot (in Ubuntu 12.04 you can directly access this using Super + A):
Click "Installed: See ... more results" to view a menu list of all installed applications.
Click to launch the application you want. Use arrow keys or the scroll bar to browse for more application down the list.
But the Dash is not limited to this... There's much more, and is always getting better!
If you want a more Windows like panel and menus, try installing UI's like Cinnamon by Linux Mint, Classic gnome in gnome3 by GNOME
For those interested in the XP-style Start Menu on Ubuntu , there is classic menu indicator ( which originally was meant to mimic old Gnome menu , which was in Ubuntu way before Unity Dash came around )
It's written by Florian Diesch , one of the esteemed members of AskUbuntu community and is available for installation via
sudo apt-get install classicmenu-indicator