When I first learned Esperanto, I learned that country names usually take the suffix -ujo to convert from a culture to its respective country (e.g. Anglujo, Francujo, Germanujo). But after using the language and talking with people around the world, I have noticed that they use another suffix -io for the countries (e.g. Anglio, Francio, Germanio).
Why is this difference? Do they mean the same, or they actually mean something else?
There is no difference in meaning betweeen -ujo ans -io in country names that are derived from a demonym (a word referring to a person's nationality).
Zamenhof used -ujo, whereas -io was introduced by Hector Hodler (the founder of UEA) in 1918. In 1922 the Lingva Komitato (the predecessor of Akademio de Esperanto) advised against the usage of -io, but as its usage grew nevertheless, the Akademio de Esperanto decided to undo this advice against -io in 1974. This decision in 1974 did however neither make -io official nor recommended, but just tolerated.
While in the 1990s, -ujo was almost dying out, it is having a comeback in the last 10-20 years. The main reason for this is that the use of -io in country names derived from demonyms is causing confusion to learners of Esperanto, as there are also many country word roots (like Aŭstrali/ and Ĉili/) that end in i and can thus be confused for a derived country name (see my answer to a related question). If one uses -ujo for the derived country names, one cannot confuse them with these non-derived country names ending in -io. The distinction is important for knowing what the demonym is (the demonym corresponding to Ĉinujo/Ĉinio is ĉino, but the demonym corresponding to Ĉilio is ĉiliano).
For this reason, the course on lernu! teaches mainly the forms with -ujo, and many highly regarded Esperanto speakers (e.g. many members of the Akademio de Esperanto) use only -ujo in country names derived from demonyms.
See the similar question Why are country names in Esperanto so "irregular"?.
The suffix -uj- is used in the Fundamento, while -i- is a newer addition. But they mean the same in the context of country names. The difference is that -i- is used only for country names, and -uj- is also used for containers (monujo = purse/wallet, not money-land!). Some people prefer -uj- because it's in the Fundamento, some -i- because they think the idea of a container of Englishmen or Frenchmen or Germans is strange, or just because they think it sounds better, and some use whichever they feel like at the moment.
One difference is that -ujo is only used to form a country name from the name of an ethnic group, while -io can also be used to form a country name from e.g. a city or a river. For example, "Alĝerio" is the country whose capital city is "Alĝero".
This is mentioned in PMEG, which however recommends not adding this suffix unless the resulting names are used internationally. A counter-example: "Senegalo" is both a country and a river, and while "Senegalio" unambigously refers to the country, that form is not used in any other language, and should therefore be avoided.
