I'm a software tester in an organisation which has clear delineation between manual testers, software test engineers who build automated tests and developers (I gotta say that I speak for me and not them here - that's the policy out of the way). The first two of that list need to have a 'question everything' testers' mentality; the last two need to have a technical grasp of how the whole thing works, from the core OS, servers and transports to the libraries used and the UI.
A manual tester is concerned with the human experience of your software. Typically there will be a script of the exact actions or the class of actions the manual tester is expected to cover. Some firms roll regression testing and new feature testing into the manual tester's role, others have a separate user-acceptance testing team who stand between released software and deployment.
The automation tester's role is to make the computer test the software, by building a framework around the program and its installed environment to check the system as-a-whole does the intended job. Some automation testers also write unit tests in the core program code; mostly this is left to the developers, who are increasingly being asked to show that their code works - and doesn't break the existing code - before check-in. Some places operate a Behavior-Driven Development (BDD) process where client needs are expressed in testable ways - so that the evidence that your software does what a client wants is written down clearly for the client to see, the tester to test and the developer to build. Some BDD systems even build tests which show that these goals are met, and are known as Test-Driven because the proof of rightness is shown by the tests.
Preparing for an telephone screening or interview about software test engineer, you will need to show that you're technically-minded, that you've worked engineered a test system before, that you understand the benefits and shortcomings of building such a system (such as knowing when to build tests and when to leave low-hanging test fruit to manual testers), and that you're concerned for the quality of the product more than for building a new toy test rig.
I disagree with nearly every point made above, but I know this as a tester: there are a lot of people who shuffle into testing because it's the gateway to working in software. That's no bad thing. But mix it with the lack of confidence to apply for and get developer roles, and you will probably harm your sanity and your career. In every job, you need to be competent and confident. On top of that, you need to be personable so, for example, you don't make bug reports critical of the developers who made them (and know how to calm down a dev who's got that idea).
To be a good tester is genuine hard work requiring the best of your technical insight, pedantry and commitment to high standards, but to be a great tester will call for charisma, kindness, honesty, good working relationships, and the skills to help make a dysfunctional team work right.
If you're only a tester for a short while and move on to a developer role, you can either blame the economy for not having enough dev jobs (so you compromised in order to fulfill your passion for making software), or you can say that you wanted to improve your developer skills by learning what it's like to be on the other side of the fence as a tester. It's not a black mark if working as a tester makes it a benefit to you and a prospective employer.