df.groupby(['col1'])['col2'].mean()  # or .count(), etc.

# other way for multiple statistics
df.groupby(['col1', 'col2'])[['col3', 'col4']].agg(['min', 'max', 'sum'])

# choose specific columns
df1.groupby('col1').agg({'col2':'count'})

# use filter to count rows
df.groupby('col1').filter(lambda x: len(x) > 1)