The summary files include canonical election-cycle data for candidates and committees - one record for each candidate or committee per two-year election cycle, depending on the file selected. There are two candidate summary files -- one for campaigns that have elections in the current cycle, and one for all candidates no matter if they face election in the cycle or not -- and they can differ in amounts and timeliness.
The current campaigns file may be more timely, but also contains a single total for PAC contributions (compared to totals for different kinds of PACs in the other file) and some of its totals may contain double-counted transactions. More at the data dictionary.
The all candidates file can be updated slightly less frequently than the current campaigns one, but it contains more detailed breakdowns of certain types of transactions as noted above. The possibility of double-counting some kinds of transactions - transfers to and from authorized committees of a candidate - also exists. More at the data dictionary.
The PAC summary file provides the latest summary information on political action and party committees, including independent expenditures. More at the data dictionary.
The FEC previously used to generate summary files at the end of the election cycle for candidates and PACs that included totals for different types of PACs, but discontinued these files after the 2005-06 cycle. Files are available from 1979-80 through 2005-06. Party committee-only summary files exist from the 1991-92 cycle through the 2003-04 cycle. One of the most useful files, which contained a record for each combination of candidate recipient and PAC contributor/independent spender, covers the 1991-92 cycle through the 2001-02 cycle. A similar file exists for candidate-party activities during the same time period.
These summary files had been stored in fixed-width format but were converted to pipe-delimited format in late July 2012.
There's one more thing to be aware of: the way that the FEC used to store its data (detailed and summary) relied on the use of an "overpunch" character to represent negative amounts. This recently changed for the detailed contribution files and for the candidate and committee files, but it's possible that older summary files still contain such characters, and that the amount fields should be imported as text and then converted to numeric columns, accounting for negative amounts. There is a tutorial for working with the FTP files using Microsoft Access.