The Senate is formed of 100 elected senators, two from each state. Every three years, a third of the senate is elected, giving each senator a term of six years. In the 2014 election, the Republicans achieved their first majority in the Senate in 8 years (54-R, 46-D). However, current polling suggests that will be short lived as the Democrats are on track to regaining their majority.
This election also favours the Democrats as it is at the same time as the presidential election so Democratic voters are more likely to turnout when compared to midterm elections. People also tend to vote for the same party as their presidential nominee, so as the polls stand, Hillary Clinton's lead should favour the Democrats in Congress.
With a depressed turnout for the Democrats, it's possible that there could be a 50-50 tie in the Senate. In this case, the Vice President would break a tie so the winner of the Senate would be the party that wins the presidency.
Every state is worth one seat (one senator).