It is in the infinitive form.
As the link above shows, it could also be +ing; the two forms are used interchangeably, although there could be different connotations as explained in the link above.
An infinitive inside a sentence does not always require "to" before it. For example, you will certainly remember the phrase "to make somebody do something". It is actually a rule that you don't use "to" here; because many people feel that it is the infinitive sense and try to use "to". This doesn't mean that, if "to" is absent, it stops being an infinitive. With other verbs, "to" is optional, so you can say "help to" and "help (without to)" interchangeably: "Would you come over and help me move some boxes?" – "Yesterday, I went to my friend's house and helped him to move some heavy boxes."