ONLINE/ON LINE/IN LINE
The common adjective used to label Internet activities is usually written as one word: “online”: “The online site selling banana cream pies was a failure.” But it makes more sense when using it as an adverbial phrase to write two separate words: “When the teacher took her class to the library, most of them used it to go on line.” The hyphenated form “on-line” is not widely used; but would be proper only for the adjectival function. However, you are unlikely to get into trouble for using “online” for all computer-related purposes.
As for real physical lines, the British and New Yorkers wait “on line” (in queues), but most Americans wait “in line.”
List of errors