Since this record is fairly modern (1957) it seems likely those references are similar to, or the same as, the modern visa categories.
Most of the entries on that page have a C-1 visa, which is a transit visa that only permits immediate onward travel to another country, and all of these have a destination in Canada, which is consistent with that.
B-2 is the normal pleasure tourist category for entry. B-1 is the business visitor version, and the visa required for both these is a "B" visa. The Philpotts from the UK in that image have that category, despite having a destination in Canada. This could be either because they plan to stop off at places in the US on the way, or (more likely) they already have a US tourist visa from an earlier trip so don't need the more restrictive transit visa.
To confirm that visa categories haven't changed much, if at all, since 1957, on page 58 of that document (Ancestry, subscription required) there are some A-2 visas (diplomatic or official business) going to a military base in the US. There's also an O-1 visa ("extraodinary ability") for William L Fuller on that page, plus the O-2 and O-3 dependent visas for his family.
Page 60 has a G-2 visa for Joan L. Robinson. The G-2 visa is for representatives of a recognized government traveling to the U.S. temporarily to attend meetings of a designated international organization and her destination is indeed shown as the UK delegation to the United Nations, New York.
In summary, many visa categories do appear to be largely unchanged since 1957. The most readable and complete summary of present-day categories is on Wikipedia.