VLOOKUP(lookup_value,table_array,col_index_num,range_lookup)
is the pseudocode you are looking for. For this specific question the below code should work:
=VLOOKUP(A1,Sheet1!A:B,2,0)
Explanation of components:
lookup_value: value to search in the first column of the array
table_array: the range of cells in which the table exists. the first column of this table is what is looked for by lookup_value
col_index_num: the column in the table in which to search, 1 = first column, 2 = 2nd column
range_lookup: used for testing of exact matches or approximate matches