This tag should be used only on questions that are about Objective-C features or depend on code in the language. The tags [cocoa] and [cocoa-touch] should be used to ask about Apple's frameworks or classes. Use the related tags [ios] and [osx] for issues specific to those platforms.

Syntax

Objective-C is a thin layer on top of C, and moreover is a strict superset of C; it is possible to compile any C program with an Objective-C compiler, and to freely include C code within an Objective-C class. Objective-C derives its object syntax from Smalltalk. All of the syntax for non-object-oriented operations (including primitive variables, pre-processing, expressions, function declarations, and function calls) are identical to that of C, while the syntax for object-oriented features is an implementation of Smalltalk-style messaging.
References

Good resources for learning Objective-C (Stack Overflow post).
Programming with Objective-C
Concepts in Objective-C Programming
Cocoa Memory Management Programming Guide, an essential read, especially the part about Ownership Policy
Learn Objective-C from Cocoa Dev Central, an introductory tutorial to the language
Object-Oriented Programming with Objective-C
WWDC 2012 videos on Modern Objective-C and Migrating to Modern Objective-C
Advanced Memory Management Programming Guide
Learn Objective-C basics from CodeSchool
Ry’s Objective-C Tutorial
Objective-C Guide For Developers
Objective-C Tutorials
Objective-C Cheat Sheet and Quick Reference