I'd say that every co-worker is a colleague, but not every colleague is a co-worker.

The usage depends on context. Within a company, my co-workers would be the people on my team (and likely, people that do a similar job to mine).

When talking to friends about my job, I could refer to all people at the company as my co-workers.

Colleague either is someone you work with in the same team, department or company (again, depending on context) but it can also be someone who works in the same industry or who has a similar job.

The prime minister met his European colleagues at the summit this Wednesday.

In this case, he met prime ministers from European countries. Not people he works with in the same team or office or government.

In that context, the words counterpart is sometimes used when we refer to a specific colleague:

The British prime minister met with his German counterpart on Friday.

Again, these two are not co-workers, but the are colleagues.

If I send out an e-mail within my company to invite you to have a drink with your colleagues, I mean your co-workers; people that work at the same company, maybe even ones in different jobs.

If I create a facebook page where, say, Java programmers can meet their colleagues, it means people that all share the same (kind of) job, not co-workers.

Colleague implies same level or type of job

In contrast to other answers that describe colleague as more general (because it applies also to similar positions in other companies), I perceive that there's a different aspect where co-worker is more general. Namely, different levels or types of jobs - if you're working as, say, a designer in company, then the company's CFO and janitor are your co-workers but describing them your colleagues can be done, but is a bit misleading.

A co-worker is someone who happens to work for the same employer as you do. No other connection is implied.

A colleague is someone with whom you have a working relationship irrespective of employer. As others have pointed out, heads of state at a G8 summit are colleagues even though they certainly work for different countries. They do, one hopes, work together to achieve common goals. That's what makes them colleagues.

The adjective, collegial, which comes from the same root, implies a degree of camaraderie. Hope this helps.

A co-worker is one who works in the same workplace, paid by the same employer; a colleague is someone doing a similar job. But there has recently been an upsurge particularly in shops and supermarkets of signage such as "ask a colleague for help" or on doors "Colleagues only", referring to "staff". These are signs aimed at customers, who are surely NOT colleagues of the staff. Evolution of the English language?

It seems to me that a colleague is a person who shares a same degree...MD, LMFT, Teacher, Professor, Engineer, Nurse. Whether they work with you or not....example Professors of psychology are continuating their research and thus all professors of psycholgy have a current specialty in common. All people you work with are co workers but not all are colleagues.

A co-worker is anyone who may work in and for the same company but has a different scope of work than what specific degreed colleagues would have assigned to them. In that sense, a co-worker would be dissimilar to a colleague.

However, once the co-worker is asked to facilitate the colleague in the production of an end result to any given task then he/she becomes a colleague for the duration of the assigned project. It is not the person who makes for a co-worker or a colleague but the entire team assigned to a specific task.

An example of this would be a surgical team where Doctors and Nurses all together assist the patient's medical outcome. At the point they are all working towards the same outcome then they are all colleagues. This notion can and should be applied to all company, medical, educational etc. pursuits for the beginning to end time of which they are working on the same outcome as a team to accomplish results of the same spectrum.

I agree with those who've said that co-worker is more general. As a teacher, I would refer to other teachers as my colleagues, but custodial staff, security, etc. as co-workers.

Interestingly, I would not refer to our principal as a colleague OR a co-worker, as neither seems to be a good fit.

I work in a hospital and a part of the administrative team. I consider that team my colleagues, because we are of the same rank. Everyone else is a co-worker. My colleagues would also be other administrative personnel in other industries. We are on the same level. That is the true definition. I hear people say everyone is their colleague but that is incorrect.