Basic just denotes that the language is accessible to beginners and is not directly held to specific standards. For instance, it's not like we get Ansi-BASIC like we do with Ansi-C. Ansi would be a documented standard that some applications adhere to, there may well be standards for the BASIC dialect but I've never seen anything like that enforced.
It's best to assume that BASIC is a very thin standard to allow for specifics, like graphics engines etc. You could probably list the core, standard commands of most BASIC's on an a4 sheet, but then there could be 1000 non-standard commands on top of that - it just depends on what language you use.
It's worth noting that some languages take the BASIC name and run off with it, often sticking it onto languages that behave more like hybrids. For instance I see PureBASIC as being quite similar to Pascal, and DBPro feels more like it has a scripting engine on top due to the readability of the commands.
The important thing is to learn the commands, so the easier the syntax, the better. I think DBPro has the most 'english' commands out of any BASIC language, so it's easy to learn because the commands are quite memorable.
