An object as a theoretical construct is very simple: It's a structure that stores data, functions, or both. The idea being that these structures have a sense of "self" which is implicit in most languages outside of Python. This is called a "descriptor" and gives the object a point of self reference that binds data (variables or fields) and functions (typically called methods) to the particular object in question. The idea being that you are specifically using a variable or method that belongs to that particular instance (allocated block of memory typically underneath) rather than to some larger, more general construct.
Object systems tend to vary with regards to two big categories: Inheritance and Access.
Some, like Java or C++, have you declare classes that act as "blueprints" for objects that then become allocated. These classes and their objects cannot be structurally modified once instantiated. They can have their contents overridden in the sense that variables can change but their structure is static. You can't, for example, add new methods to HashMap in Java. You can extend the interface (basically partially implemented classes serving as contracts) or make a subclass to get the extra methods or variables you need along with all of the original variables and methods of the particular class in question.
Other class based languages, the most typically cited one being Ruby, allow you to easily open up an existing class and just add in methods as you see fit. This is a bone of contention and considered by many to be very, very dangerous.
Access control is the other big point of difference between the various languages.
Some languages like Java have very strictly enforced access modifiers like "private" and "protected", that define exactly what classes and subclasses can use a given variable or method.
Others, such as Python are less formal, using the convention of an underscore before the method or variable name to indicate that it is private.
Ultimately, Python is a perfectly legitimate language to program in an object-oriented fashion, it just doesn't enforce it quite as rigorously as some of the others.