Research Associate
The title of research associate is used to denote an academic research position at a university or similar institution. A research associate usually conducts research under the supervision of a principal investigator. In contrast to a research assistant or research officer, a research associate often has a doctoral degree or carried out an equivalent work. In some cases it can be synonymous with postdoctoral research.
http://en.wikipedia.org/wiki/Research_associate

A Research Assistant is a junior graduate, employed on a temporary contract by a college or university or a research institute, for the purpose of academic research. Research assistants are not independent and not responsible for the out-come of the research and are responsible to a supervisor or Principal Investigator.
Research assistants (in some institutions referred to as Research Officer) are often enrolled in a higher degree (e.g. Masters or Ph.D.) programme. In research institutions without graduate programmes, research assistants positions are typically for graduates who are gaining research experience on short term contracts.
http://en.wikipedia.org/wiki/Research_assistant

The title of research fellow is used to denote a research position at a university or similar institution, usually for academic staff or faculty members. A research fellow may act as independent investigator, or under the supervision of a principal investigator. In contrast to a research assistant or research officer, the position of research fellow, normally requires a doctoral degree or equivalent work.

In the United Kingdom, at many universities, this position is the first career grade of a Research Career Pathway and may be permanent, subject to normal probation regulations. Within such a path, the next two higher career grades are usually senior research fellow and professorial fellow. Although similar to the position of a research fellow, these two positions are research only posts, with the rise of the career grade there will normally be a formal requirement of a moderate amount of teaching and/or supervision (often at postgraduate level). These positions are for researchers with a proven track record of generating research income to fund themselves and producing high quality research output that is internationally recognised.

In North America, a research fellow is often a Post-doctoral fellow, who undertakes research in addition to doctoral research. Alternatively a research fellow may be a visiting research fellow from another institution, who tends to be at a more senior career level. There are also faculty research fellow (FRF), being a faculty member without tenure.

Germany
The English term research fellow is sometimes used to refer to the holder of a research fellowship from a public foundation that promotes research. Fellowships, form prestigous institutions such as Alexander von Humboldt Foundation, can be obtained by postdoctoral researchers at the beginning of their academic career, by experienced, established scholars and scientists, or even leading authorities in their discipline[1]. This means that the award holder may formally hold a specific title at his or her home institution (e.g. Privatdozent), but may in the context of the sponsor be refer to as research fellow

http://en.wikipedia.org/wiki/Research_fellow