Biomedical researchers have to efficiently explore the scientific literature, keeping the focus on their research. This goal can only be achieved if the available means for accessing the literature meet the researchers' retrieval needs and if they understand how the tools filter the perpetually increasing number of documents. We have examined existing web-based services for information retrieval in order to give users guidance to improve their everyday practice of literature analysis. We propose two dimensions along which the services may be categorized: categories of input and output formats; and categories of behavioural usage. The categorization would be helpful for biologists to understand the differences in the input and output formats and the tasks they fulfil in information-retrieval activities. Also, they may inspire future bioinformaticians to further innovative development in this field.