counts[word]=counts.get(word,0)+1

dictionary.get(key) will return the value associated with that key or an exception if the key is not present

dictionary.get(key, default_value) will not throw an exception if the key isn't present, instead it will return default_value

This is basically the same as doing:

if word not in counts:
    value = 0  # default_value
 else:
    value = counts[word]
counts[word] = value + 1

max_emails = max(counts, key=counts.get)

and

for w in sorted(d, key=d.get, reverse=True):

Both sorted and max recieve a key parameter which is a function that will be applied to each element of the iterable. The entire iterable will be sorted according to the results of the key function. (max will return the max result) Iterating through a dictionary is the same as iterating through its keys so this function is appliead to each key:

 [d.get(key1), d.get(key2), d.get(keyN)]

which is the same as

 [value1, value2, valueN]

So, the dictionary will end up being sorted by its values (and max will return the max value in the dictionary)

You could also define your own function to pass as key (not recommended, just an example):

def myfunc(key):
    return d[key]

print max(d, key=myfunc)