There are three utilities in Informix to load data to the database from flat files:
Load SQL command. Very simple to use, but not very flexible. I would recommend this for small amounts of records (less than 10k)
Dbload, which is a command line utility, a bit more complex than the load sql command. This will allow you to have more control on how the records are loaded: commit intervals, starting point in the flat file, number of errors before exiting, etc. I'd recommend this utility for a small to medium sized data loads (>10k<100k).
HPL, or High Performance Loader, which is a rather complex utility that can load data at a very high rate of speed, but with a lot of overhead. Recommended for large to x-large data loads.