nslookup usually operates in interactive mode. You can specify options like debug, d2, type= , etc. in interactive mode using the ' set' command. Those same options can be specified by using switches directly without going into interactive mode; in fact any option that can be set in interactive mode can be used in a switch:
nslookup -type=mx domain.tld nslookup -debug domain.tld
To bring up nslookup's help in interactive mode, just issue a ? (question mark). To bring up the syntax that is listed below I had to provide more than two options on the command line: nslookup 1 2 3 brought up:
nslookup [-opt ...] # interactive mode using default server
nslookup [-opt ...] - server # interactive mode using 'server'
nslookup [-opt ...] host # just look up 'host' using default server
nslookup [-opt ...] host server # just look up 'host' using 'server'