Campaign contributions are governed by state and federal statutes. For example, some state statutes require judges to step down from hearing a case where there may be the appearance of impropriety due to having received a campaign contribution from one of the litigants. In national presidential races, the Federal Campaign Finance Law places legal limits on how much and what you can give. Certain individuals, businesses, and associations are prohibited from making contributions to federal candidates or political committees.
Besides checks and currency, the FEC considers "...anything of value given to influence a Federal election" to be a contribution. This does not include volunteer work. As long as you are not compensated for it, you can perform an unlimited amount of volunteer work. Donations of food, beverages, office supplies, printing or other services, furniture, etc. are considered "in-kind" contributions, so their value counts against contribution limits.
