Some Jewish communities of the Diasporal have everything needed for a candidate to convert: a rabbinical court for conversion, defined guidelines on expected norms of behaviour, organized courses of study for the potential convert, and a mikveh for ritual immersion. Other communities may have no rabbinical court, but do offer a preparatory course of study.  Some communities, of course, have no conversion services whatsoever.

Every Jewish community that has a rabbinical court for conversion (like New York, or the Ashkenazic community in London), determines its own criteria for conversion. The creteria,  however,  such as length of study, and level of commitment required, vary from community to community.
The Israeli rabbinate recognizes only Orthodox conversions performed by a bet din (rabbinical court)  that appears on its list of recognized rabbinical courts.
For a list of recognized rabbinical courts for conversion, click here. 

Communities abroad (like the Sepharadic community in London,  many Orthodox communities in the Americas, and communities in the Former Soviet Union) that do not do perform the conversion itself, but offer only courses of study for potential converts send converts to large cities in their country, or to Israel, for their actual conversion (appearance before bet din and tevila (ritual immersion). Conversion courts in Israel are careful not to circumvent rabbis of communities abroad, and tend to consider applicants from abroad only if they qualify for immigrant status, or if they present a recommendation from the relevant rabbinic authorities of the applicant's home country. Conversion courts in Israel have greater flexibility in dealing with converts from communities that are completely lacking in any conversion process or preparation (eg. Spain, Portugal, Argentina, and India).

In order to receive Israeli citizenship after conversion abroad, the Interior Ministry requires you to live in the community in which you converted for at least one year.