The choice of a Russian dictionary depends on the level of the student and the goal of study.
Beginners should, in most cases, start with a bilingual essential Russian dictionary to be able to translate words between their mother tongues and Russian and build their initial vocabulary fast. A Russian dictionary for beginners will normally provide additional pronunciation and basic grammar tips.
A larger bilingual Russian dictionary can of course also be used by intermediate and advanced students, but at this level it makes sense for them to use a monolingual Russian dictionary in which Russian words are explained in Russian rather than just translated. The reason is simple—having to deal with as much Russian as possible improves one’s language, often subconsciously and thus effortlessly.
A monolingual essential Russian dictionary is a good choice for most students. Larger monolingual dictionaries are for proficient students and serious linguistic research.
The Russian word for dictionary is словарь (sla-VAHR)—in case you decide to look for one in Russian on the Internet.