Switching between tabs within the same Window in an app
Solution 1:
A universal keyboard shortcut is Command + { to tab towards left and Command + } to tab towards right. (You'll need to press and hold Shift key, to enter { and } key).
This shortcut works with almost all the apps that I personally use, which support multiple tabs under a single window. A few examples include: Apple's own apps such as:
Safari
Maps
Preview
Terminal
Xcode
as well as 3rd party apps such as:
Sublime Text
Google Chrome
Firefox
Opera
Solution 2:
Use Command + Shift + ← to go to the left tab and Command + Shift + → to go the to right tab.
This works in both Finder and Safari. It could be other combinations for other programs.
Solution 3:
In general, there is no Universal way to do this, unless you're content to do all your work in Safari and Finder (as @iTunes said).
Even terminal has a different scheme for switching (Command + `).
In Firefox and Chrome, it's Command + Option + ← and Command + Option + →.
Solution 4:
Control + Tab works in a lot of applications, including Safari, Firefox, Finder, and Terminal. It's also cross-platform - works on Windows as well to switch between program tabs. Holding down Shift reverses order (right-to-left instead of left-to-right).